add make build to makefile and elaborate on build steps in CONTRIBUTING - #1357.

CONTRIBUTING.md

@@ -29,16 +29,19 @@ $ cd babel
 $ make bootstrap
 ```
 
-Then you need to run:
+Then you can either run:
+
+```sh
+$ make build-core
+```
+
+to build Babel **once** or:
 
 ```sh
 $ make watch-core
 ```
 
-This will compile Babel and then sit in the background and on file modification
-recompile the necessary files. Babel itself is written in ES6. The source files
-reside in `src/` and transpile to `lib/`
-
+to have Babel build itself then incrementally build files on change.
 
 #### Running tests
 

Makefile

@@ -118,3 +118,4 @@ bootstrap:
 	cd packages/babel-cli && npm install && npm link && npm link babel-core
 	git submodule update --init
 	cd vendor/compat-table && npm install object-assign
+	make build