Merge src/leveldb changes for LevelDB 1.18.

src/leveldb/CONTRIBUTING.md

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+# Contributing
+
+We'd love to accept your code patches! However, before we can take them, we
+have to jump a couple of legal hurdles.
+
+## Contributor License Agreements
+
+Please fill out either the individual or corporate Contributor License
+Agreement as appropriate.
+
+* If you are an individual writing original source code and you're sure you
+own the intellectual property, then sign an [individual CLA](https://developers.google.com/open-source/cla/individual).
+* If you work for a company that wants to allow you to contribute your work,
+then sign a [corporate CLA](https://developers.google.com/open-source/cla/corporate).
+
+Follow either of the two links above to access the appropriate CLA and
+instructions for how to sign and return it.
+
+## Submitting a Patch
+
+1. Sign the contributors license agreement above.
+2. Decide which code you want to submit. A submission should be a set of changes
+that addresses one issue in the [issue tracker](https://github.com/google/leveldb/issues).
+Please don't mix more than one logical change per submission, because it makes
+the history hard to follow. If you want to make a change
+(e.g. add a sample or feature) that doesn't have a corresponding issue in the
+issue tracker, please create one.
+3. **Submitting**: When you are ready to submit, send us a Pull Request. Be
+sure to include the issue number you fixed and the name you used to sign
+the CLA.
+
+## Writing Code ##
+
+If your contribution contains code, please make sure that it follows 
+[the style guide](http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml).
+Otherwise we will have to ask you to make changes, and that's no fun for anyone.