docs: Document thread priority configuration.

docs/THREAD_PRIORITY.md

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+# Thread priorities
+
+Proton supports fine-grained thread priority control using `setpriority(2)` to
+set thread niceness values corresponding to the game threads' Windows base
+priority levels. However, most default Linux configurations don't allow
+individual threads to raise their priority, so some system configuration is
+likely required.
+
+It can be configured as a priviledged user by editing the
+`/etc/security/limits.conf` file, or using the `/etc/security/limits.d/` conf
+directory, and adding the following line at the end:
+
+```
+*   hard  nice    -15
+```
+
+Where -15 could be any value between [-20,0] that will correspond to the
+minimum niceness (the highest priority) a thread can get to. The lower the
+value, the more CPU time a high priority thread will get, at the expense of
+others and other processes, possibly making the system less responsive.