Document WS clientConfig options in README and RTD

[cgewecke]

Description

PR targets #3190 (because it's a Websockets improvement)

Adds documentation about how to configure the Websocket provider's data size limits. This issue was raised recently in #3388 where someone was trying to read a large block from Ropsten.

PR updates WS module README and creates a sub-section about configuring providers in the ReadTheDocs documentation.

Type of change

• Documentation

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run dtslint with success and extended the tests and types if necessary.
• I ran npm run test:unit with success.
• I have executed npm run test:cov and my test cases do cover all lines and branches of the added code.
• I ran npm run build-all and tested the resulting file/'s from dist folder in a browser.
• I have updated the CHANGELOG.md file in the root folder.
• I have tested my code on the live network.

[coveralls]

Coverage remained the same at 88.116% when pulling df8157e on pri/document-ws-provider-options into 3ab0779 on provider-related-improvements.

[holgerd77]

LGTM