README improvements

[giannisdaras]

Some issues spotted in the README file of the project:

• [x] Greek language models are not uploaded yet to spaCy. Remember to change the corresponding section.
• [x] Article for Explosion AI blog is not ready yet. Remember to change the corresponding section.
• [x] Extend/Justify more the innovative approaches .
• [x] Add images wherever possible.
• [x] Check for language/expression errors.
• [x] Write tests section when tests are ready.
• [x] Complete contribute instructions.
• [x] Check SPACE tag.

[giannisdaras]

Closed