Add CONTRIBUTING.md for guidance on contributing

CONTRIBUTING.md

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+# How to Contribute
+
+CSI is under [Apache 2.0](LICENSE) and accepts contributions via GitHub pull requests.
+This document outlines some of the conventions on development workflow, commit message formatting, contact points and other resources to make it easier to get your contribution accepted.
+
+## Markdown style
+
+To keep consistency throughout the Markdown files in the CSI spec, all files should be formatted one sentence per line.
+This fixes two things: it makes diffing easier with git and it resolves fights about line wrapping length.
+For example, this paragraph will span three lines in the Markdown source.
+
+## Git commit
+
+### Commit Style
+
+Simple house-keeping for clean git history.
+Read more on [How to Write a Git Commit Message](http://chris.beams.io/posts/git-commit/) or the Discussion section of [`git-commit(1)`](http://git-scm.com/docs/git-commit).
+
+* Separate the subject from body with a blank line.
+* Limit the subject line to 50 characters.
+* Capitalize the subject line.
+* Do not end the subject line with a period.
+* Use the imperative mood in the subject line.
+* Wrap the body at 72 characters.
+* Use the body to explain what and why vs. how.
+  * If there was important/useful/essential conversation or information, copy or include a reference.
+* When possible, one keyword to scope the change in the subject (i.e. "README: ...", "tool: ...").