Add initial README

README.md

+gluon-builder
+-------------
+
+An ffbs tool to build Gluon images in a reproducible way.
+
+builder.sh is configures via environment variables:
+
+* BRANCH: Defines which branch you want to build for.
+          Usually one of `parker_beta` or `parker_stable`.
+
+* ARCHITECTURES: Space-separated list of Gluon architectures to build.
+          If omitted will build for all architectures.
+
+* J:      Number of threads to use.
+
+Examples:
+
+* Building for Chrissi's CI:
+  `BRANCH=parker_beta J=40 ARCHITECTURES="ramips-mt76x8" ./builder.sh`
+
+* Building for Production:
+  `BRANCH=parker_stable J=40 ./builder.sh`