Document differences between tendermint/gno and mainline tendermint

[moul]

Comparisons

• Tendermint v0.34: https://github.com/tendermint/tendermint/tree/v0.34.24
• Tendermint2: https://github.com/tendermint/tendermint2

Expected output

• A full document on the tendermint2 repo.
• A link to this first document on the current repo.

Can count for #408 if done before the end of phase 1.