Update README to be more explicit

[sepulzera]

It took me longer than I would like to admit, to understand where to put the env.js. Probably should have headed directly towards the sample code.

Please update the README to include:

1. In step 2, put the env.js in src.
2. Add info when the build env.js will be generated. Step 3 misleaded me into thinking that a dummy file would be generated during the build. As it didn't, I thought I messed something up.