[readme] Fix instructions.

Thanks to Pierre Rousselin for the report!

Author: ejgallego

README.md

@@ -1,10 +1,20 @@
 # Build Instructions
 
+First you need to setup jsCoq and related tools:
+```
+make node_modules    # Install jsCoq and other tools
+```
+
+once that is done, you can work with your files as:
+
 ```
 $ make       # Build Coq files (customize makefile if you need)
-$ make serve # Install jsCoq and open a web server
+$ make serve # Open a web server with the jsCoq files
 ```
 
+Note that `make serve` doesn't need to be restarted after a regular
+`make`.
+
 # Tweaks from official distro
 
 We have customized the following files from the oficial distro: