add basic README for :distribution

this is primarily to document the JDK & No JDK setup (to codify it for
the future), however the README can be extended in the future to
document more information which is currently hidden away only somewhere
in the code.

Signed-off-by: Ralph Ursprung <Ralph.Ursprung@avaloq.com>

distribution/README.md

@@ -0,0 +1,12 @@
+# Distributions
+This subproject contains the necessary tooling to build the various distributions.
+Note that some of this can only be run on the specific architecture and does not support cross-compile.
+
+The following distributions are being built:
+* Archives (`*.zip`, `*.tar`): these form the basis for all other OpenSearch distributions
+* Packages (`*.deb`, `*.rpm`): specific package formats for some Linux distributions
+* Docker images
+* Backwards compatibility tests: used internally for version compatibility testing, not for public consumption
+
+## With or Without JDK?
+For each supported platform there should be both a target bundled with a JDK and a target without a bundled JDK.