Update README.md

Added closing parenthesis on the first sentence of the License section.
Merge pull request #1914 from glennon/patch-1

Update README.md
iwyu
svg_renderer: use precision of 4, accept map request/scale_factor, and fix output of multipart geometries
spirit::karma based to_hex_string generator
+ to_string based on spirit::karma for completeness.
catch boost::filesystem::directory_iterator::construct error and print warning rather than throwing exception - closes #1919 - refs mapbox/tilemill#1992
+ fix typo in grammar
iwyu