add basic readme for bundle specifications

build/README-bundle.md

@@ -0,0 +1,31 @@
+# Builtin Actor Bundles
+
+With NV16, builtin actor bundles must be loaded into lotus for the FVM to operate.
+
+The bundles are specified in build/bundles.toml using the foloowing syntax:
+```toml
+[[bundles]]
+version = X   # actors version
+release = tag # release gag
+```
+
+This will add a bundle for version `X`, using the github release `tag`
+to fetch the bundles at first startup.
+
+If you don't want to fetch the bundle from github, you can specify an explicit path to the bundle (which must be appropriate for your network, typically mainnet):
+```toml
+[[bundles]]
+version = X   # actors version
+release = tag # release gag
+path = /path/to/builtin-actors.car
+```
+
+For development bundles, you can also specify `development = true` so that the bundle is not
+recorded in the datastore and reloaded every time the daemon starts up:
+```toml
+[[bundles]]
+version = X   # actors version
+release = tag # release gag
+path = /path/to/builtin-actors.car
+development = true
+```