[FEATURE] - Improve documentation (README) to tell about its dependency to the language engine

[Alessandro Fragnani]

A few bug issues were reported, telling the extension wasn't drawing the separators, but in the end, the reason was the language extension itself, which didn't work. After telling the user how the extension works and to validate if the language extension were providing the symbols, the issues could be closed (#31 and #25).

In order to properly warn the user how the extension works, it's a good idea to add such info on README, providing details and guidance on how to validate/report issues on this matter