Merge branch 'refs/heads/t/doc'

The contributing section for the user manual and a few other
documentation updates.

* refs/heads/t/doc:
  signal.h: Add documentation of signal_pre_select().
  manual: Fix kernel coding style link.
  user manual: Add section on contributing.

diff --cc NEWS.md
index 3d00528d123929d78d4895a5a57fd748c085cdad,a7af771744e91e3654bd216b217f40a56eae11ea..d47141ce1f8112cdd73b59814b9124dad5dd5019
--- 1/NEWS.md
--- 2/NEWS.md
+++ b/NEWS.md
@@@ -15,7 -15,6 +15,8 @@@ NEW
  - The wma decoder and audio format handler now correctly decodes
    files with unusual block sizes.
  - We now compile with -Wformat-signedness if possible.
 +- The touch command now refuses to set an invalid image or lyrics ID.
++- New section on contributing for the user manual.
  
  Download: [tarball](./releases/paraslash-git.tar.bz2)