(First) Revise the Readme

[BraINstinct0]

Yes, considering that this has been released just a few days ago I won't expect the documentation to be fully written at this point.
However, it seemed to me that if users were given the choice to write down what they'll need, a 'good' readme would be easier to write.
Leaving a few notes here as a starter:

• a short and concise Installation of tabnine-jupyterlab (and which is visible at first sight!)
• a warning related to the project still in alpha/beta status
• too much promotional (note: I agree that tabnine is good, but it's just too much for a Readme :)) parts should be reduced. Linking to the website seems enough.