Default branch renamed from "master" to "develop"

[mdpiper]

After consulting with @mcflugen, we've renamed the default branch of the BMI repository to develop, which we hope more accurately reflects the purpose of this branch. I've submitted a PR (#114) adding a note to the README recommending implementers use the latest tagged release, not the tip of the develop branch.

If you have a local clone of the BMI repository, you can update it by running the following commands:

git branch -m master develop
git fetch origin
git branch -u origin/develop develop
git remote set-head origin -a