Add cross-doc links, avoid redundancies

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

.github/issue_template.md

@@ -2,11 +2,12 @@ _Note:_ this is a template, please remove the parts that are not
 applicable (these initial notes, and the "Bug" section for a Feature request
 and vice-versa).
 
-**Note:** to report a security vulnerability, see `SECURITY.md`. Please do not
-use github issues for vulnerabilities.
+**Note:** to report a security vulnerability, see
+[SECURITY.md](../SECURITY.md). Please do not use github issues for
+vulnerabilities.
 
-_Note:_ To get support, see `SUPPORT.md`. Please do o't use github issues for
-questions.
+_Note:_ to get support, see [SUPPORT.md](../SUPPORT.md). Please do not use
+github issues for questions.
 
 ---------------------------------------------------------------
 ### Description

BRANCHES.md

@@ -30,13 +30,17 @@ will put security first but provide a compatibility option. (So far we never
 had to break ABI compatibility in an LTS branch, but we occasionally had to
 increase code size for a security fix.)
 
-## Currently maintained branches
+For contributors, see the [Backwards Compatibility section of
+CONTRIBUTING](CONTRIBUTING.md#cackwords-compatibility).
+
+## Current Branches
 
 The following branches are currently maintained:
 
-- development (2.x.y releases)
-- Mbed TLS 2.16, maintained until at least the end of 2021, see
+- [development](https://github.com/ARMmbed/mbedtls/)
+- [mbedtls-2.16](https://github.com/ARMmbed/mbedtls/tree/mbedtls-2.16)
+ maintained until at least the end of 2021, see
   <https://tls.mbed.org/tech-updates/blog/announcing-lts-branch-mbedtls-2.16>
-- Mbed TLS 2.7 - end of life in March 2021!
+- [mbedtls-2.7](https://github.com/ARMmbed/mbedtls/tree/mbedtls-2.7) - end of life in March 2021!
 
 Users are urged to always use the latest version of a maintained branch.

CONTRIBUTING.md

@@ -22,9 +22,10 @@ Making a Contribution
 1. All new files should include the [Apache-2.0](https://spdx.org/licenses/Apache-2.0.html) standard license header where possible.
 1. Ensure that each commit has at least one `Signed-off-by:` line from the committer. If anyone else contributes to the commit, they should also add their own `Signed-off-by:` line. By adding this line, contributor(s) certify that the contribution is made under the terms of the [Developer Certificate of Origin](dco.txt). The contribution licensing is described in the [License section of the README](README.md#License).
 
-API/ABI Compatibility
----------------------
-The project aims to minimise the impact on users upgrading to newer versions of the library and it should not be necessary for a user to make any changes to their own code to work with a newer version of the library. Unless the user has made an active decision to use newer features, a newer generation of the library or a change has been necessary due to a security issue or other significant software defect, no modifications to their own code should be necessary. To achieve this, API compatibility is maintained between different versions of Mbed TLS on the main development branch and in LTS (Long Term Support) branches.
+Backwards Compatibility
+-----------------------
+
+The project aims to minimise the impact on users upgrading to newer versions of the library and it should not be necessary for a user to make any changes to their own code to work with a newer version of the library. Unless the user has made an active decision to use newer features, a newer generation of the library or a change has been necessary due to a security issue or other significant software defect, no modifications to their own code should be necessary. To achieve this, API compatibility is maintained between different versions of Mbed TLS on the main development branch and in LTS (Long Term Support) branches, as described in [BRANCHES.md](BRANCHES.md).
 
 To minimise such disruption to users, where a change to the interface is required, all changes to the ABI or API, even on the main development branch where new features are added, need to be justifiable by either being a significant enhancement, new feature or bug fix which is best resolved by an interface change.
 
@@ -48,6 +49,9 @@ When backporting to these branches please observe the following rules:
 
 It would be highly appreciated if contributions are backported to LTS branches in addition to the [development branch](https://github.com/ARMmbed/mbedtls/tree/development) by contributors.
 
+The list of maintained branches can be found in the [Current Branches section
+of BRANCHES.md](BRANCHES.md#current-branches).
+
 Currently maintained LTS branches are:
 1. [mbedtls-2.7](https://github.com/ARMmbed/mbedtls/tree/mbedtls-2.7)
 1. [mbedtls-2.16](https://github.com/ARMmbed/mbedtls/tree/mbedtls-2.16)

README.md

@@ -25,6 +25,8 @@ To generate a local copy of the library documentation in HTML format, tailored t
 1. Run `make apidoc`.
 1. Browse `apidoc/index.html` or `apidoc/modules.html`.
 
+For other sources of documentation, see the [SUPPORT](SUPPORT.md) document.
+
 Compiling
 ---------
 

SUPPORT.md

@@ -2,7 +2,8 @@
 
 Here are some useful sources of information about using Mbed TLS:
 
-- API documentation (see `make apidoc` or directly the header files);
+- API documentation, see the [Documentation section of the
+  README](README.md#License);
 - the `docs` directory in the source tree;
 - the [Mbed TLS knowledge Base](https://tls.mbed.org/kb);
 - the [Mbed TLS mailing-list