List issues with rebasing in README Caveats / Limitations

[Erotemic]

I think it important to be more upfront about issues with rebasing like those brought up in #8. These limitations should be clearly stated in the README, so fools like myself don't almost push all their secrets in plaintext.

[jmurty]

I'm pretty confident that the underlying merge issue is fixed in the pending PR #76 that adds a merge driver to better handle conflicts that occur during rebasing, or merging in general.

[jmurty]

The merge and rebase problems should be solved by PR #76 which is now merged to the master branch and will be included in the next release.