Add short howto for release process

RELEASE_HOWTO.md

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+# How to cut a new release
+
+1.  Make sure you are on the right branch, usually master
+2.  Make sure everything that needs to be committed is committed
+3.  Pick a version number.
+    * Bugfixes: final digit
+    * New features visible to user: middle digit
+    * New way to deploy: middle digit
+4.  Update the version number in the file `package.json` first, easy to forget.
+    It's close to the top of the file.
+5.  Run `npm install` to upgrade the `package-lock.json`-file
+6.  Update the changelog with `towncrier build --version yournewversionnumber`.
+    Feel free to massage the generated text!
+7.  Commit the changes from steps 4 to 6 in the same commit.
+8.  Tag the commit made in step 7 with an annotated tag with the new version
+    number.
+
+    ```
+    git tag -a -m "Mainly bugfixes" v1.5.1
+    ```
+
+    Sum up the most important changes in the release in the max. 50 characters
+    long message.
+9.  Push everything to github
+10. Make a new release on github, this is needed for some automation that
+    generates a new version in other repos.