Add Dart client to README and clients page.

[marceloneppel]

This change is 

[CLAassistant]

All committers have signed the CLA.

[danielmai]

Congrats on releasing this new Dart client for Dgraph, @katutz. Have you thought about giving the client library a different name other than plain "dgraph"?

Thanks for your contribution!

[marceloneppel]

Congrats on releasing this new Dart client for Dgraph, @katutz. Have you thought about giving the client library a different name other than plain "dgraph"?

Thanks for your contribution!

Thanks @danielmai! I thought in using some other name like ddgraph or dgraph_dart. But I was in doubt, because I saw some clients in Dart with the same name as theirs databases and others with a different name. Then I initially used the same name to reserve the dgraph package name in the Dart Pub (Dart package repository). Would it be interesting to switch to one of these names I thought or some other?