Merge pull request #1675 from KostyaSha/contributing

Add CONTRIBUTING.md file.

CONTRIBUTING.md

+# Contributing to Jenkins
+For information on contributing to Jenkins, check out the https://wiki.jenkins-ci.org/display/JENKINS/contributing and https://wiki.jenkins-ci.org/display/JENKINS/Extend+Jenkins wiki pages over at the official https://wiki.jenkins-ci.org . They will help you get started with contributing to Jenkins.
+

README.md

@@ -10,7 +10,7 @@ Non-source downloads such as WAR files and several Linux packages can be found o
 Our latest and greatest source of Jenkins CI can be found on [GitHub]. Fork us!
 
 # Contributing to Jenkins
-For information on contributing to Jenkins, check out the [Contributing] and [Extend Jenkins] wiki pages over at the official [wiki]. They will help you get started with contributing to Jenkins.
+Follow [contributing](CONTRIBUTING.md) file.
 
 # News and Website
 All information about Jenkins CI can be found on our [website]. Follow us on Twitter [@jenkinsci].