Document how to run a testnet node

[leto]

Describe the issue

We need docs on how to run a testnet node, or else nobody will know how.

Can you reliably reproduce the issue?

Yes

If so, please list the steps to reproduce below:

1. No
2. Docs
3. TO RTFM

Expected behaviour

To be able to RTFM

Actual behaviour + errors

No RTFMing possible

The version of Hush you were using:

ALL

[lludlow]

• Create TESTNET.md
• Update README.md with link to TESTNET.md

See PR #17

[leto]

This is nicely documented in https://github.com/MyHush/hush/blob/dev/Testnet.md now, I think we just need to add a link to it from README.md

[dawqfoundation]

not safe for newbies