Add prerequisite information to README

Trying to save people the heartache I had to go through when trying to figure out why grunt-pages wasn't working for me.
normalize page paths for pageSrc pages before generating destination path.

There was an issue when slicing abspath based on the length of roodir.
It would only work if pageSrc was defined at a level deeper than the grunt intance.
For example, `pageSrc: '../pages'` would cause broken destination names, e.g. "ex.html" instead of "index.html"
Using node's 'path' module, the path can be normalized for comparison.

Not sure if you want to write unit tests for these cases. Might be worthwhile to use the built-in 'files' as opposed to custom options.

Note that this is probably an issue other places where you are treating filepaths as strings.
Merge branch 'master' of github.com:justinhelmer/grunt-pages

Conflicts:
	README.md