Added a contributors section to README.md

In preparation for accepting GitHub pull requests this new README
section outlines the general criteria that the leveldb project owners
will use when accepting external (and internal) project contributions.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=111349899

README.md

@@ -22,6 +22,37 @@ Authors: Sanjay Ghemawat (sanjay@google.com) and Jeff Dean (jeff@google.com)
   * Only a single process (possibly multi-threaded) can access a particular database at a time.
   * There is no client-server support builtin to the library.  An application that needs such support will have to wrap their own server around the library.
 
+# Contributing to the leveldb Project
+The leveldb project welcomes contributions. leveldb's primary goal is to be
+a reliable and fast key/value store. Changes that are in line with the
+features/limitations outlined above, and meet the requirements below,
+will be considered.
+
+Contribution requirements:
+
+1. **POSIX only**. We _generally_ will only accept changes that are both
+   compiled, and tested on a POSIX platform - usually Linux. Very small
+   changes will sometimes be accepted, but consider that more of an
+   exception than the rule.
+
+2. **Stable API**. We strive very hard to maintain a stable API. Changes that
+   require changes for projects using leveldb _might_ be rejected without
+   sufficient benefit to the project.
+
+3. **Tests**: All changes must be accompanied by a new (or changed) test, or
+   a sufficient explanation as to why a new (or changed) test is not required.
+
+## Submitting a Pull Request
+Before any pull request will be accepted the author must first sign a
+Contributor License Agreement (CLA) at https://cla.developers.google.com/.
+
+In order to keep the commit timeline linear
+[squash](https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History#Squashing-Commits)
+your changes down to a single commit and [rebase](https://git-scm.com/docs/git-rebase)
+on google/leveldb/master. This keeps the commit timeline linear and more easily sync'ed
+with the internal repository at Google. More information at GitHub's
+[About Git rebase](https://help.github.com/articles/about-git-rebase/) page.
+
 # Performance
 
 Here is a performance report (with explanations) from the run of the