Update the README and Rakefile to reflect current process of generating docs, fixes #1802

* The bits about generating docs in the ember.js and
ember-data directories should be safe to remove since
the generate_docs rake task takes care of generating docs
in both of those folders.

* In the Rakefile, 'node bin/generate_docs.js' is not a
valid command in the ember.js repo anymore, so this replaces it
with 'npm run docs'.

* This commit also addresses a few minor typos.
Merge pull request #1803 from tehcurtis/update_readme

Update the README and Rakefile to reflect current process of generating ...