Add git/github guidelines

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Git and GitHub Guidelines

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**Write good commit messages** that at least follow the spirit of

:ref:`this example <commit-msg>`.

**Write many small commits** each containing an atomic change instead of one

large mega-commit. This not only makes code review easier, but it also makes

commits that show up in ``git blame`` 10 years from now make more sense.

Strive to **write many small commits** each containing an atomic change instead

of one large mega-commit. This not only makes code review easier, but it also

makes commits that show up in ``git blame`` 10 years from now make more sense.

**Prefer a rebase workflow instead of merge**. Incorporating PRs should be done

with fast-forward merge, if easily possible. The larger the topic branch, the

harder this may be, so merge commits are allowed.

If, while working on a topic branch, some changes are made to master that

conflict with your work or that you need to incorporate into your work, **do

not merge master into your topic branch**; instead, rebase your topic branch on

top of master or cherry-pick the changes.

**Do not force push lightly** unless branches are clearly labeled as ones that

may get overwritten (for example: "transient\_" prefix). Instead of overwriting

a branch, add a version suffix (for example: "_02").

Coding Guidelines

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