Added README to explain how to keep the eigen mirror in sync.
diff --git a/README.md b/README.md
index 4654a81..d33c3dd 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,68 @@
 **Eigen is a C++ template library for linear algebra: matrices, vectors, numerical solvers, and related algorithms.**
 
 For more information go to http://eigen.tuxfamily.org/.
+
+Mirroring Eigen
+===============
+
+This repository is a mirror of the C++ linear algebra library **Eigen**.
+
+Because Eigen is hosted on BitBucket in a Mercurial repository, you will need
+a way to convert between mercurial and git. We use git-remote-hg for this
+purpose.
+
+``https://github.com/felipec/git-remote-hg``
+
+Check out this repository.
+
+``git clone https://fuchsia.googlesource.com/third_party/eigen``
+``cd eigen``
+
+Add a remote pointing to the mercurial bucket on bitbucket.
+
+``git remote add bitbucket hg::https://bitbucket.org/eigen/eigen``
+``git fetch bitbucket``
+
+Now, we need to update the branches we care about. As of the writing of this
+document, we only care about upstream/master (the Eigen tip of tree)  and
+upstream/3.3 (the 3.3.x line of stable releases).
+
+Track the branch on fuchsia.googlesource.com.
+
+``git checkout --track origin/upstream/master``
+
+Fetch the changes from bitbucket.org and upload.
+
+``git pull bitbucket master``
+``git push origin upstream/master``
+
+Repeat the process for the other branches we care about.
+
+Bumping the Version Used In Fuchsia
+===================================
+
+As you noticed, the master branch on fuchsia.googlesource.com is not the same
+as the master branch on bitbucket. That's because our master branch is kept at
+a stable release.
+
+To see if you need to update the master branch, see if a new stable version is
+available.
+
+``git log -1``
+
+Look for "tag: 3.3.4" or similar on the first line. This is version at which
+fuchsia is building eigen.
+
+``git ls-remote bitbucket``
+
+And see if there is a tag for the new version.
+
+If there is, check it out on the master branch.
+
+``git checkout master``
+``git reset --hard 3.3.5``
+
+And push to fuchsia.
+
+``git push origin 3.3.5``
+``git push origin master``