Include links to Plasma implementations

[smartcontracts]

Questions:

• Should we link to any available implementation or just actively maintained?
• Can someone find a list of implementations?

[smartcontracts]

Some links, from ethresear.ch:

https://github.com/wolkdb/deepblockchains/tree/master/Plasmacash

https://github.com/voltairelabs/plasma

https://github.com/omisego/plasma-mvp

https://github.com/omisego/plasma-cash

https://github.com/loomnetwork/plasma-erc721

https://github.com/FourthState

https://github.com/ethereum-plasma/plasma

https://github.com/BANKEX/PlasmaParentContract

[maxweng]

Let's put all links here. I'm also working on a Plasma DEX https://github.com/FastXProtocol/plasma-mvp

[barrasso]

I will include a "Known Implementations" list in the docs. I think we should only link to actively maintained implementations to minimize noise and maximize code reliability.

[smartcontracts]

Reopening to get some more links

[tpmccallum]

LayerXcom's Vyper implementation of Plasma https://github.com/LayerXcom/plasma-mvp-vyper