Update CONTRIBUTING.md file

Signed-off-by: Antoine Leblanc <[email protected]>

Author: antleblanc

.github/CONTRIBUTING.md

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+# Contributing to gulp-drupal-stack
+
+This project accepts contributions. In order to contribute, you should
+pay attention to a few things:
+
+1. your code must follow the coding style rules
+2. your code must be unit-tested
+3. your code must be documented
+4. your work must be signed (see below)
+5. you may contribute through GitHub Pull Requests
+
+# Submitting Modifications
+
+The contributions should be submitted through Github Pull Requests
+and follow the DCO which is defined below.
+
+# Licensing for new files
+
+gulp-drupal-stack is licensed under a MIT license. Anything
+contributed to gulp-drupal-stack must be released under this license.
+
+# Developer Certificate of Origin (DCO)
+
+To improve tracking of contributions to this project we will use a
+process modeled on the modified DCO 1.1 and use a "sign-off" procedure
+on patches that are being emailed around or contributed in any other
+way.
+
+The sign-off is a simple line at the end of the explanation for the
+patch, which certifies that you wrote it or otherwise have the right
+to pass it on as an open-source patch.  The rules are pretty simple:
+if you can certify the below:
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I have
+    the right to submit it under the open source license indicated in
+    the file; or
+
+(b) The contribution is based upon previous work that, to the best of
+    my knowledge, is covered under an appropriate open source License
+    and I have the right under that license to submit that work with
+    modifications, whether created in whole or in part by me, under
+    the same open source license (unless I am permitted to submit
+    under a different license), as indicated in the file; or
+
+(c) The contribution was provided directly to me by some other person
+    who certified (a), (b) or (c) and I have not modified it.
+
+(d) The contribution is made free of any other party's intellectual
+    property claims or rights.
+
+(e) I understand and agree that this project and the contribution are
+    public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.
+
+
+then you just add a line saying
+
+    Signed-off-by: Random J Developer <[email protected]>
+
+using your real name (sorry, no pseudonyms or anonymous contributions.)