update readme instructions

README.md

@@ -123,7 +123,7 @@ You can download a pre-built binary version of rebar3 based on the last commit f
 
 https://s3.amazonaws.com/rebar3/rebar3
 
-#### Building rebar
+#### Bootstrapping rebar3
 
 ```sh
 $ git clone https://github.com/rebar/rebar3
@@ -131,6 +131,15 @@ $ cd rebar3
 $ ./bootstrap
 ```
 
+### Developing on rebar3
+
+When developing you can simply run `escriptize` to build your changes but the new escript is under `_build/default/bin/rebar3`
+
+```sh
+$ ./rebar3 escriptize
+$ _build/default/bin/rebar3
+```
+
 
 Contributing to rebar
 =====================