README: Change req->request, res->response, other fixes

[cllns]

In our full-stack guides we use request, response instead of the shorter req, res. I think this is helpful for teaching people, since it's more clear.

I found and fixed some other issues while going through it too:

1. Fixed the typo: res.ression → response.session (line 540)
2. Fixed the escaped wildcards: "*/*" → "/" (line 764)
3. Fixed the spelling: alterative → alternative (line 666)
4. Fixed the method name consistency: resource.update_at → resource.updated_at (lines 677, 678, 681)
5. Fixed the garbled line: gle() → def handle() (line 335)
6. Fixed the abbreviation: eg → e.g. (line 99)
7. Cleaned up extra blank line and improved alignment in the MIME type example (lines 781-782)