Update docs and readme for `unstable` branch

[Tbaut]

It's currently outdated, what stands out from the top of my head is

• add/reject token and network https://github.com/ChainSafe/dappeteer/issues/181
• sign typed data #191
• new bootstrap params and return object https://github.com/ChainSafe/dappeteer/pull/167

[mpetrunic]

Another thing we should add is:

• example how to write a test for snap (initSnapEnv, testDappPage)

[Tbaut]

done in #202