add library pros and cons to README

[SethTisue]

Sometimes people stumble upon this library and figure that if it it's in the scala org it must be the bee's knees. I think we should foreground the reasons to choose it or not.

[SethTisue]

@j-mie6 @johnynek LGTY? is anything major missing from the "weaknesses" section? accent on "major" — that section shouldn't be too long and needn't be comprehensive

[j-mie6]

I'd say that:

• The API is a little disorganised and missing a few niceties that I'd expect from a modern PC library (like precedence combinators)
• it doesn't seem to have any way of customising the error messages/formatting other than raw messages (including the output type being more structured than String)
• it doesn't have any natural integration with something like cats? (perhaps that's too minor)

[SethTisue]

Those are good observations, but maybe a bit too detailed. I added a bullet that says "Minimal feature set" in an attempt to cover it.

[j-mie6]

Perhaps "Rigid error messages" might be enough high-level for point 2?

[SethTisue]

Added: "Inflexible, unstructured error reporting"

(Incidentally, I wouldn't be opposed to linking to external resources with more detailed comparison info.)

[SethTisue]

merging... we can always change it further, whenever. thanks for the help