[DOC] Update a "contrib" file with a hint about a scheme used for formathing subjects

With each new patch I had to search for the e-mail from Willy
describing the schem used for formathing subjects. No more. ;)

CONTRIB

@@ -7,3 +7,25 @@ Special thanks to the following people for their useful contributions :
 - Marcus Rueckert : support for username/groupname.
 - Bryan Germann : option to exempt some sources from x-forwarded-for.
 - Fabrice Dulaunoy : health-checks on different addresses
+
+Note to contributors: it's very handy when patches comes with a properly
+formated subject. Try to put one of the following words between brackets
+to indicate the importance of the patch followed by a short description:
+
+[MINOR]    minor fix, very low risk of impact
+[MEDIUM]   medium risk, may cause unexpected regressions of low importance or
+           which may quickly be discovered (just like this patch)
+[MAJOR]    major risk of hidden regression. This happens when I rearrange large
+           parts of code, when I play with timeouts, with variable
+           initializations, etc...
+[BUG]      fix for a minor or medium-level bug.
+[CRITICAL] medium-term reliability or security is at risk, an upgrade is
+           absolutely required.
+[RELEASE]  release a new version
+[BUILD]    fix build issues. If you could build, no upgrade required.
+[CLEANUP]  code cleanup, silence of warnings, etc... theorically no impact
+[TESTS]    added regression testing configuration files or scripts
+[DOC]      documentation updates, no need to upgrade
+[LICENSE]  licensing updates (may impact distro packagers)
+
+Example: "[DOC] document options forwardfor to logasap"