Rework CONTRIBUTING.adoc
Rework CONTRIBUTING.adoc

Several fixes were applied: Syntax corrected, links updated, improved some paragraphs, restored parts that were removed in the conversion to adoc format (see: a8b438587b129d455f4c2c93f556fd6c080a19aa)

jgrandja/spring-security,djechelon/spring-security,spring-projects/spring-security,jgrandja/spring-security,jgrandja/spring-security,djechelon/spring-security,spring-projects/spring-security,jgrandja/spring-security,fhanik/spring-security,djechelon/spring-security,fhanik/spring-security,fhanik/spring-security,jgrandja/spring-security,spring-projects/spring-security,spring-projects/spring-security,spring-projects/spring-security,rwinch/spring-security,rwinch/spring-security,djechelon/spring-security,rwinch/spring-security,rwinch/spring-security,rwinch/spring-security,jgrandja/spring-security,djechelon/spring-security,fhanik/spring-security,spring-projects/spring-security,fhanik/spring-security,rwinch/spring-security,spring-projects/spring-security,fhanik/spring-security