Add instructions for building with Bazel?

[tfarina]

Update README file to also include info on how to build with using Bazel @mzhaom? Thanks!

[floitsch]

We are moving our Dart SDK to bazel soon. I hope I will get more exposure to bazel then.
Once I'm a bit more comfortable with bazel I will update the README.

[smahapatra1]

bazel build //... gives the following error:

double-conversion/bignum-dtoa.cc:30:10: fatal error: double-conversion/bignum-dtoa.h: No such file or directory
#include <double-conversion/bignum-dtoa.h>
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.

I think bazel expects you to quote them as "double-conversion/bignum-dtoa.h".