Documentation for fees

[izyak]

What needs to be done

Document the fees required for all the tokens for the cross chain token transfer.

Why it needs to be done

There is no documentation for how the fee works and what fee is required for cross chain transfer of tokens in the repo.

Stories

Currently, the only way to get the fee is to query the BTS contract for getFee method for required token name.

Acceptance Criteria

Maintain a list of tokens and current fee required to transfer them,