Add guide on how to run explorer for older contract version

[kelsos]

Explorer page is tied to the raiden-contracts version.

This means that if someone is using an older version of the client for any reason they have no access to the explorer for that version of the network.

There should be a guide provided in the explorer page, that a user can follow so they can run a local version (dockerized) of the explorer with the deployed contracts version they need.

This should contain:

• How to find out the version of the contracts you are interested in.
• Instructions on how to modify the setup with the proper contracts version.
• Instructions on how to build, start and access your local explorer instance

[karlb]

Using older contracts versions is easy now by setting EXPLORER_CONTRACTS_VERSION as in

explorer/docker-compose.yml

Line 27 in a402582

- EXPLORER_CONTRACTS_VERSION=0.37.0

.