Add changelog for PKCS7 parser

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>

ChangeLog.d/pkcs7-parser.txt

@@ -0,0 +1,13 @@
+Features
+   * Added partial support for parsing the PKCS7 cryptographic message syntax,
+     as defined in RFC 2315. Currently, support is limited to the following:
+     - Only the signed data content type, version 1 is supported.
+     - Only DER encoding is supported.
+     - Only a single digest algorithm per message is supported.
+     - Only 0 or 1, certificate is supported per message, which must be in
+       X509 format.
+     - There is no support for certificate-revocation lists.
+     - The authenticated and unauthenticated attribute fields of SignerInfo
+       must be empty.
+     Many thanks to Daniel Axtens, Nayna Jain, and Nick Child from IBM for
+     contributing this feature.