Add CS_DMG_HEADSHOT define

but i don't know if it works in other games.

( I'm using this already in my only headshot plugin - https://forums.alliedmods.net/showthread.php?t=234169 and works fine in CSS/CSGO )
Merge pull request #7 from Bara20/patch-1

Add CS_DMG_HEADSHOT define (r=psychonic).
Create CONTRIBUTING.md

As discussed in #11.
Merge pull request #15 from 50Wliu/contributing

Create CONTRIBUTING.md (r=psychonic).
Fix inconsistencies with OnClientDisconnected calls (bug 5988)
Merge pull request #16 from psychonic/client-disconnect-quirks

Fix inconsistencies with OnClientDisconnected calls (bug 5988, r=kyles)