Docs on multi context build.

Author: chmac

nr-server/README.md

@@ -1 +1,12 @@
-# nostroots-server
+# nostroots-server
+
+This is a deno app that runs on the server to watch for 30397 events, check that
+they have a valid NIP05 on trustroots.org, and then post a matching 30398 event.
+
+In order to load `nr-common` into docker, we use a multi context build. To build
+it locally you can run:
+
+    docker buildx build --build-context nr-common=../nr-common .
+
+This exposes the `nr-common` folder as `nr-common` inside docker, hence the
+first lines of `Dockerfile`.