Links to the range syntax of version numbers

This has been very unclear.

Some people don't know that ranges can be specified for version numbers because it hasn't been mentioned in the `bower.json` section of the readme.

Other people notice that ranges are mentioned in the "Installing packages" of the readme, but have not a slightest idea of the syntax (i tried the `Gemfile` syntax and it didn't work, and http://semver.org doesn't describe ranges).

Example of the ambiguity: https://github.com/bower/bower/issues/505#issuecomment-37511088