Fix typos and inaccuracies in README

[dylanrainwater]

1. Line 47: "the the Rails" -> "the Rails"
2. Line 122: "has changed" -> "have changed"
3. Line 153: "a application.css" -> "an application.css"
4. Line 177: "not recommend" -> "not recommended"
5. Line 256: "load the file up one directory and named" -> "load a file named foo.js from one directory up" (awkward phrasing)
6. Line 399: "can use .. to ." -> "can use .. to reference parent directories." (incomplete sentence)
7. Line 429: "tresolved" -> "resolved"
8. Line 472: "your must" -> "you must"
9. Lines 491-492: Fixed ERB syntax - changed <% to <%= and fixed quote conflicts in the examples
10. Lines 517, 562, 583: Changed incorrect path app/application/ to app/assets/ (three occurrences for javascripts and stylesheets directories)