Add library.json for use with PlatformIO

[danielschenk]

By adding this file, the library can be added to PlatformIO's library
registry. This way, projects built with PlatformIO can depend on json11
and it will be automatically downloaded from GitHub and linked with the
final program.

[artwyman]

Thanks for the PR, @danielschenk. I don't know anything about PlatformIO, so am not sure what opinion to have on this one. It's certainly non-dangerous, but I'm not sure about the precedent of adding files to the repo to support arbitrary third-party tools and frameworks. I'm not sure, for instance, whether other tools may have different uses for the filename library.json which we might ruling out by merging this.

So far this repo supports build config for make and cmake, both of which are widely used standards in the C++ community. We didn't ever end up adding config files for gyp, even though Dropbox uses it internally to build this into our mobile apps. I'd be curious to hear the level of community interest in this PR.

Logistically, @danielschenk note that no PRs can be merged until the author has signed the CLA at https://opensource.dropbox.com/cla/