Merge #12852: [doc] devtools: Setup ots git integration

fa385c3 [doc] devtools: Setup ots git integration (MarcoFalke)

Pull request description:

  Document the simple steps on how to set up ots git integration.

Tree-SHA512: 1b9f99bfaa6cd9dc581243d3a3584301645e95450acc3b5898dcdb53849569de16bb8ef2676b18f6b8dd402de10aee80119e15c1b28cef36f17ad121cbba2ba3

Author: laanwj

contrib/devtools/README.md

@@ -144,6 +144,14 @@ Configuring the github-merge tool for the bitcoin repository is done in the foll
     git config githubmerge.testcmd "make -j4 check" (adapt to whatever you want to use for testing)
     git config --global user.signingkey mykeyid (if you want to GPG sign)
 
+Create and verify timestamps of merge commits
+---------------------------------------------
+To create or verify timestamps on the merge commits, install the OpenTimestamps
+client via `pip3 install opentimestamps-client`. Then, dowload the gpg wrapper
+`ots-git-gpg-wrapper.sh` and set it as git's `gpg.program`. See
+[the ots git integration documentation](https://github.com/opentimestamps/opentimestamps-client/blob/master/doc/git-integration.md#usage)
+for further details.
+
 optimize-pngs.py
 ================