add cleanup script

This gets rid of unwanted files in a repository.

cleanup-2-x-x-mess.sh

@@ -0,0 +1,12 @@
+#!/bin/bash -eu
+main()
+{
+	local templateDir=$(git config --get --path init.templatedir)
+
+	if [ ! -d .git ]
+	then
+		echo "This script is supposed to be run at the root of a git repository" >&2
+	fi
+	rm -rf .git/tests .git/docs .git/mkdocs.yml .git/README.md .git/LICENSE .git/CONTRIBUTING.md .git/update.sh
+}
+main

docs/upgrade/2-to-3.md

@@ -1,5 +1,7 @@
 # Upgrading from 2.x.x to 3.x.x
 
+## Changing the template directory
+
 There is just a little change between 2 and 3, it is the location of the template
 directory. It is no longer the root in the project, but a sub-directory named…
 `template`. Why this change ? Because even if the update script can ignore the
@@ -14,3 +16,11 @@ To change that, upgrade to 3.x.x, and adjust your `init.templatedir` variable:
 
 After that you will be able to safely run the update script from any project you
 have.
+
+## Cleaning up repositories
+
+If you do not like the mess I have created in your repositories, please:
+
+- accept my sincerest apologies
+- run the cleanup script I wrote: `/path/to/.git_template/cleanup-2-x-x-mess.sh`
+from the root of each repository you want to clean.