dist: tools: coccinelle: add documentation

dist/tools/coccinelle/README.md

+## Overview
+
+Coccinelle allows testing C code against semantic patches, automatically
+creating diffs on the way. See http://coccinelle.lip6.fr/ for more information.
+
+## Coccinelle in RIOT
+
+RIOT uses Coccinelle as part of the CI static-tests as follows:
+
+- every added or modified file will be checked
+- there are two sets of coccinelle patches: "force" and "warn", found in
+  dist/tools/coccinelle/(warn|force)
+- if coccinelle has a hit in "warn", the resulting patch will be printed, but
+  doesn't cause the CI to fail
+- if coccinelle has a hit in "force", the CI build fails
+
+We add semantic patches first to "warn", fix our codebase, and if the patch is
+confident enough, move it to "force".
+
+If coccinelle is installed, you can test the whole RIOT checkout by executing
+
+    $ dist/tools/coccinelle/check.sh
+
+... or just test a single patch:
+
+    $ dist/tools/coccinelle/check.sh path/to/semantic_patch.cocci
+
+... or just the files changed to <branch>:
+
+    $ BRANCH=<branch> dist/tools/coccinelle/check.sh
+
+... or combined:
+
+    $ BRANCH=<branch> dist/tools/coccinelle/check.sh path/to/semantic_patch.cocci
+
+... or test a single file:
+
+    $ FILES=path/to/file dist/tools/coccinelle/check.sh [path/to/semantic_patch.cocci]
+
+## Applying coccinelle's patches:
+
+If the coccinelle diff makes sense and you'd like to apply it, you can do so by
+piping coccinelle's output into patch:
+
+    $ dist/tools/coccinelle/check.sh | patch -p0
+
+Specifying BRANCH, FILES and/or a specific patch file works here, too.
+
+Beware that some of our semantic patch files might just *find* occurences of
+bad code or style! Not all semantic patches output useful diffs.
+
+## My PR has coccinelle warnings/errors!
+
+Using above commands, try to find out which file caused the warning/error, and
+why.  If you're absolutely sure your code is correct, you can ignore it if it's
+a  warning.  It's advisable to get this confirmed by others.
+
+If the PR triggers a coccinelle error on code that you're sure is correct, open
+a PR that moves the corresponding patch from dist/tools/coccinelle/force to
+dist/tools/coccinelle/warn.