fix(options): Add buffer sizes to failover. Update README

[ndyakov]

• add missing buffer size to failover options
• half the default buffer size
• add section in readme related to buffer sizes

Fixes #3465

[htemelski-redis]

Looks, good there are two minor things to change but they are already noted

[ofekshenawa]

LGTM

[cxljs]

Hello @ndyakov , Here's a small suggestion: How about keeping the read/write buffer size of the sentinel client at 4KB? Since there's no need for the sentinel client to handle large data transfers or pipeline scenarios.

[ndyakov]

@cxljs great idea! Feel free to work on that or open a separate issue so we can pick it up when possible.

[cxljs]

OK, I'll submit a PR later.