Add a guide for solana wallet adapter.

[Saumay Agrawal]

Hi!

I'd like to make an addition to the guide section. I wish to add a tutorial guide for solana wallet adapter. But I can't see any community contribution guidelines on this repo.

I already have the content prepared here: https://www.solanawalletadapter.xyz/

How do I go about adding it here so that more devs are able to make use of it?

Thanks!