Add gluon repository as submodule

Instead of recording the commit in the gluon repository that belongs to
a release of our firmware in the README.md, it is more reasonable to
have the gluon repository as a submodule, having this information in a
machine-readable way and making it easier to reproduce the compilation
of a specific version.

To compile gluon, simply add `GLUON_SITEDIR="$(dirname $PWD)"` to the
make command.

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "gluon"]
+	path = gluon
+	url = https://github.com/freifunk-gluon/gluon.git

gluon

@@ -0,0 +1 @@
+Subproject commit e54e3dc41046d9334d515f87397c436451c964ad