contributing: Add branching how-to

doc/development/branching_how-to.md

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+# Branching How-To
+
+## Assumptions
+
+Given the creation of a new release branch will typically happen right
+before RC1 of a new release series, please see assumptions in the
+`howto_release.md` document.
+
+## Create a New Branch
+
+Given how the branch protection rules are set up and work,
+you need to bypass protection againts merge commits on branches
+(we don't want any new ones, but there are existing from the past
+which prevet creation of a new branch).
+To bypass, go to _Settings > Rules > Rulesets > Rules for release branches_.
+Press _Add bypass_ and add the team or user who is creating the branch.
+
+Use GitHub web interface to create a new branch:
+
+1. Go to _branches_.
+2. Copy name of one of the existing branches.
+3. Click _New branch_.
+4. Paste name of the existing branch.
+5. Modify the name.
+6. Click _Create branch_.
+
+As an alterntaive to creation in GitHub, you can create a new branch using
+command line (instructions included at the end of the document).
+
+Note down the latest commit hash on the branch which will be recorded in the
+[release history overview](https://grass.osgeo.org/about/history/releases/).
+The instructions for updating the website come later in the procedure.
+
+Remove the bypass in _Settings_.
+
+## Check the Version
+
+```bash
+git fetch upstream
+git switch releasebranch_8_4
+```
+
+## Increase Version on the main Branch
+
+The version number needs to be increased on the main branch.
+
+```bash
+git switch main
+git fetch upstream
+git rebase upstream/main
+```
+
+Update the version in the source code (use `minor` or `major`):
+
+```bash
+./utils/update_version.py minor
+```
+
+If you are using a clone you use for building GRASS GIS,
+clean up (`make distclean`) your GRASS GIS build to remove
+the now outdated generated version numbers.
+(You don't need to build GRASS GIS if you have a fresh clone.)
+
+Search for all other mentions of last few versions to see if they need to be updated,
+for example:
+
+```bash
+grep --exclude-dir=.git -IrnE "[^0-9^a-z]8[\._][0-9][^0-9]"
+grep --exclude-dir=.git -IrnE "grass8.?[0-9]"
+```
+
+After the check and update, commit 
+
+```bash
+git switch -c update-version
+git add -p
+./utils/update_version.py suggest
+git commit -m "version: ..."
+git push
+```
+
+Create a PR and review and merge it as soon as possible to avoid having
+the wrong version on the branch in case other PRs need to be merged.
+
+## Server Updates
+
+In the grass-addons repo:
+
+* Add new branch into [`.github/workflows/ci.yml`](https://github.com/OSGeo/grass-addons/blob/grass8/.github/workflows/ci.yml).
+
+## Website Updates
+
+Add the branch creation to the following history:
+
+<https://github.com/OSGeo/grass-website/blob/master/content/about/history/releases.md>
+
+## Additional Notes
+
+### Making Changes in General
+
+If you make changes, commit them:
+
+```bash
+git commit -m "version: ..."
+git push
+```
+
+(You can directly push to release branches.)
+
+### Create by Command Line
+
+Get the latest main branch:
+
+```bash
+git switch main
+git fetch upstream
+git rebase upstream/main
+```
+
+Create the branch:
+
+```bash
+git switch -c releasebranch_8_3
+```
+
+Push the version to the upstream repo:
+
+```bash
+git push upstream