Create workflow/branches.md document [skip ci]

- explain 3 long living branches and their relations.
- describe a simple bugfix workflow

docs/workflow/branches.md

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+# Branches
+
+PowerShell has a number of [long-living](https://git-scm.com/book/en/v2/Git-Branching-Branching-Workflows#Long-Running-Branches) branches
+
+* **master** -- current development, upstream. It's based on source-depot, but has changes for Linux/OS X. 
+It also has changes for the latest version of .NET Core, i.e. rc3.
+* **source-depot** -- an exact mirror of dev branch in Source Depot. 
+Here code flow in "sd -> github" case. 
+It should be treated mostly as **read-only** (unless you are integrating changes "sd->github").
+* **server2016** -- branch with the code that we want to ship in Server 2016. We use it for "github -> sd" integration.
+
+### Example bugfix
+
+Create a [feature-branch](https://git-scm.com/book/en/v2/Git-Branching-Branching-Workflows#Topic-Branches) from **server2016**.
+
+```
+> # switch branch to server2016 to create feature-branch from the right commit
+> git checkout server2016
+Branch server2016 set up to track remote branch server2016 from origin.
+Switched to a new branch 'server2016'
+
+# create feature branch
+> git checkout -b vors/encoding
+```
+
+**Note** how we use `alias/feature-name` pattern in the example above.
+
+Then we develop the changes in the feature branch. 
+We can push it to the [`origin`](https://github.com/PowerShell/PowerShell) to kick-in CI build or share work-in-progress.
+
+```
+> git push origin vors/encoding
+```
+
+Eventually we merge feature branch back to **server2016** via a Pull Request with a codereview from my peers.
+
+After merging feature branch we should bring changes into **master**, so we have bugfixes in the upstream branch as well.
+
+```
+> git checkout master
+> git checkout -b vors/master
+> # look-up the sha1 for relevant commits (i.e. with gitk --all)
+> git cherry-pick <sha1>..<sha2>
+> git push origin vors/master
+```
+
+**Note** the first commit is not included in the diaposone, so the interval is open on the left side.
+
+Then I can create a Pull Requst from `vors/master` to `master` via GitHub web interface.