add readme

README.md

@@ -0,0 +1,18 @@
+Build
+=====
+
+    docker-compose run --rm builder npm run build:chrome
+
+or
+
+    docker-compose run --rm builder npm run build:firefox
+
+Signed
+======
+
+Needs AMO API key from Mozilla.
+
+    docker-compose run --rm \
+        -e AMO_JWT_ISSUER="$USER" \
+        -e AMO_JWT_SECRET="$SECRET" \
+        builder npm run package:firefox-signed