Fix typos in README.md

[slel]

File -> Filing, hestitate -> hesitate

[coveralls]

Coverage remained the same at 63.054% when pulling 454dcc3 on slel:patch-2 into 91fb468 on dcjones:master.

[shashi]

this should have remained "File"?

[slel]

Which one sounds correct?

File an issue is extremely valuable in helping me prioritize what to work on.
Filing an issue is extremely valuable in helping me prioritize what to work on.

[tlnagy]

You're right. I wasn't careful when reading, the second sounds better. It isn't the best worded sentence.

[slel]

The same sentence is at the end of http://gadflyjl.org which uses
https://github.com/dcjones/Gadfly.jl/blob/master/doc/index.md
(that page has a number of typos too, I'll send a pull request).

[slel]

In fact, the sentence was correct up until this commit,
arising from a misunderstanding of its syntax:
f8ec68d

[tlnagy]

Don't worry about the docs, I'm redoing most of it in #879

[tlnagy]

Or open a PR against that branch if you're interested.

[slel]

Okay. Maybe splitting the long sentence, that some found confusing,
into shorter sentences, as below, would decrease confusion:

This is a fairly complex piece of software.

Please [file an issue] to report any bug or counterintuitive behavior,
or even to request a feature.

This is extremely valuable in helping prioritize what to work on, so don't hesitate.

(with a link from "file an issue" to https://github.com/dcjones/Gadfly.jl/issues).