ION install docs: Add requestMaxRetries config parameter

[thobson88]

TODO:

Update the docs to add the following parameter in the ION bitcoin config file $ION_BITCOIN_CONFIG_FILE_PATH:

"requestMaxRetries": 6

Reason:

The ION synchronisation step (when npm run bitcoin runs for the first time) takes several hours and appears to store everything in memory until the end (when it writes the results of parsing all blocks to the mongodb). It's also quite common for an RPC block request to fail and (by default) if three fail then the process stops and has to be restarted from the beginning.

This change makes that less likely.

The requestMaxRetries parameter gets picked up in the Sidetree BitcoinProcessor class and passed to BitcoinClient.