Document the new CI formatting check in README.md

README.md

+# dune-gfe
+
+This module contains implementations of various geometric finite element methods (GFE).
+
+
+## Automatic code formatting
+
+The GitLab CI system of this module (in the file `.gitlab-ci.yml`) includes a job
+that checks for proper code formatting.   The CI system
+uses the tool [uncrustify](https://github.com/uncrustify/uncrustify) to enforce the rules, guided by
+the configuration file `dune-uncrustify.org` in the `dune-gfe` module source directory.
+
+CI testing of the code formatting means that any merge request will fail CI
+testing if it introduces code changes that violate the formatting rules.
+To check locally whether code changes that you have made comply with the rules,
+run
+```
+uncrustify -l CPP -c dune-uncrustify.cfg --no-backup `find -name "*.cc" -o -name "*.hh"`
+```
+in your source directory.  If this does not modify the code, then your formatting
+is correct.
+
+When starting to use `uncrustify` to enforce proper code formatting, one large commit
+had to be made that applied large amounts of white-space changes to pretty much
+every source file.  If you are bothered by this when using `git blame`, you can
+tell `git` to ignore this particular commit by saying
+```
+git blame --ignore-revs-file=.git-blame-ignore-revs <file>
+```