Update README.md
Merge https://github.com/less/less.js into misleading-error-message-2069

Conflicts:
	README.md
Raise error in case of an empty selector inside :extend.
Merge https://github.com/less/less.js into misleading-error-message-2069

Conflicts:
	test/sourcemaps/basic.json
Merge pull request #2120 from SomMeri/misleading-error-message-2069

Misleading error message 2069