Adding contribution instruciton readme from legal.

CONTRIBUTING.md

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+# Contributing Code
+
+External contributors will be required to sign a Contributor's License Agreement. You can find this file [here](Salesforce_CLA.pdf). Please sign, scan, and send to [osscore@salesforce.com](mailto:osscore@salesforce.com). We look forward to collaborating with you!
+
+1. Create a new issue before starting your project so that we can keep track of what you are trying to add/fix. That way, we can also offer suggestions or let you know if there is already an effort in progress.
+2. Fork off this repository.
+3. Create a topic branch for the issue that you are trying to add. When possible, you should branch off master.
+4. Edit the code in your fork.
+5. Send us a well documented pull request when you are done. 
+
+The **GitHub pull requests** should meet the following criteria:
+
+  - descriptive title
+  - brief summary
+  - @mention several relevant people to review the code
+  - add helpful GitHub comments on lines that you have questions / concerns about
+
+We'll review your code, suggest any needed changes, and merge it in. Thank you.