Add contributing instructions.

Change-Id: I9fe9bd339f2aacb04394259ee8753dbc8e041430
Reviewed-on: https://swiftshader-review.googlesource.com/5340
Reviewed-by: Shannon Woods <shannonwoods@google.com>
Tested-by: Nicolas Capens <capn@google.com>
diff --git a/CONTRIBUTING.txt b/CONTRIBUTING.txt
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+Want to contribute? Great! First, read this page (including the small print at the end).

+

+### Before you contribute

+Before we can use your code, you must sign the

+[Google Individual Contributor License Agreement]

+(https://cla.developers.google.com/about/google-individual)

+(CLA), which you can do online. The CLA is necessary mainly because you own the

+copyright to your changes, even after your contribution becomes part of our

+codebase, so we need your permission to use and distribute your code. We also

+need to be sure of various other thingsĀ—for instance that you'll tell us if you

+know that your code infringes on other people's patents. You don't have to sign

+the CLA until after you've submitted your code for review and a member has

+approved it, but you must do it before we can put your code into our codebase.

+Before you start working on a larger contribution, you should get in touch with

+us first through the issue tracker with your idea so that we can help out and

+possibly guide you. Coordinating up front makes it much easier to avoid

+frustration later on.

+

+### Code reviews

+All submissions, including submissions by project members, require review.

+

+### The small print

+Contributions made by corporations are covered by a different agreement than

+the one above, the

+[Software Grant and Corporate Contributor License Agreement]

+(https://cla.developers.google.com/about/google-corporate).
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