CB-6491 add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to Apache Cordova
+
+Anyone can contribute to Cordova. And we need your contributions.
+
+There are multiple ways to contribute: report bugs, improve the docs, and
+contribute code.
+
+For instructions on this, start with the 
+[contribution overview](http://cordova.apache.org/#contribute).
+
+The details are explained there, but the important items are:
+ - Sign and submit an Apache ICLA (Contributor License Agreement).
+ - Have a Jira issue open that corresponds to your contribution.
+ - Run the tests so your patch doesn't break existing functionality.
+
+We look forward to your contributions!