openwrt: flesh-out the documentation a bit

README.md

@@ -27,7 +27,7 @@ TL;DR:
 ```json
 # /etc/docker/daemon.json
 {
-    "storage-driver": "overlay2"
+	"storage-driver": "overlay2"
 }
 ```
 
@@ -60,10 +60,38 @@ Filesystem                 Size  Used Avail Use% Mounted on
   submakefile in `openwrt/` will deal with actually collecting inputs
   and building.
 
+- Set up the diffconfig and file tree for the target you want to
+  build. (The `openwrt/menuconfig-%` target wraps the menuconfig
+  OpenWrt target for this purpose; see the next point.)
+
+- Run `make` on the right (.PHONY) target to build the necessary
+  containers and e.g. build OpenWrt. The target name is based on the
+  action, remote and revision you want; with `$action` being output,
+  menuconfig or debug.
+
+- The full name format is: `openwrt/${action}-${remote}_${revision}`,
+  where `$remote` is in `openwrt-remotes.txt`, and with `$revision`
+  being e.g. a tag, a branch, etc. at the remote you use.
+
+- Additional files can be added under `openwrt/files/`.
+
+- Example run:
+
 ```sh
 cd ${docker_src}/;
+make openwrt/menuconfig-mirror_21.02
+# Wait for build and go through menuconfig
+# Save the OpenWrt .config file to the default location, then quit.
+# Once you quit, the diffconfig will be collected and dropped to output/diffconfig.
+
+cp openwrt/output/diffconfig mr16-for-testing.diffconfig
 cp mr16-for-testing.diffconfig openwrt/diffconfig
+mkdir -p openwrt/files/etc
+touch openwrt/files/etc/hello # This will end up in the image at /etc/hello
+
+# With the diffconfig + files in place:
 nohup make openwrt/output-mirror_21.02 &
-# output at ...
+
+# Watch the output at:
 tail -f openwrt/output/screen.log
 ```