KAFKA-2321; Introduce CONTRIBUTING.md

Author: Ismael Juma <ismael@juma.me.uk>

Reviewers: Guozhang

Closes #97 from ijuma/kafka-2321 and squashes the following commits:

4834464 [Ismael Juma] KAFKA-2321; Introduce CONTRIBUTING.md

CONTRIBUTING.md

@@ -0,0 +1,11 @@
+## Contributing to Kafka
+
+*Before opening a pull request*, review the [Contributing](http://kafka.apache.org/contributing.html) and [Contributing Code Changes](https://cwiki.apache.org/confluence/display/KAFKA/Contributing+Code+Changes) pages.
+
+It lists steps that are required before creating a PR.
+
+When you contribute code, you affirm that the contribution is your original work and that you
+license the work to the project under the project's open source license. Whether or not you
+state this explicitly, by submitting any copyrighted material via pull request, email, or
+other means you agree to license the material under the project's open source license and
+warrant that you have the legal authority to do so.

build.gradle

@@ -60,6 +60,7 @@ rat {
   excludes.addAll([
     '**/.git/**',
     'build/**',
+    'CONTRIBUTING.md',
     'gradlew',
     'gradlew.bat',
     '**/README.md',