clarify body-parser and LocalStrategy in readme

[zrisher]

This PR adds a bit more detail to the readme around two major stumbling blocks for new users:

• It's unclear where LocalStrategy in the first code example comes from
• It's unclear that body-parser is required for fields to be available in req.body, where passport-local expects them.

I haven't seen anyone complaining about the former, but it was a bit confusing for me and easy enough to fix as part of this PR.

There's plenty of examples of people having issues with the latter:

• Should mention the dependency of body-parser in the doc #143
• Issue with debugging when username/password cannot be parsed from request #129
• http://stackoverflow.com/questions/18690354/passport-local-strategy-not-getting-called
• http://stackoverflow.com/questions/24688142/nodejs-passport-authenticate-callback-not-being-called-with-no-errors

[coveralls]

Coverage remained the same at 97.619% when pulling 3282b88 on zrisher:master into 3b9980f on jaredhanson:master.

[gabrielcsapo]

👍 this is so needed, quick glance would be so helpful with this information!

bump @jaredhanson

[rwky]

Merged in https://github.com/passport-next/passport-local