Lack of sentinel support needs to be prominent

[ChrisMoth]

Description

node-redis is a solid and performant client - in a word fantastic.

My nodejs project was upended when our IT team was unable to deploy my application due to lack of sentinel support.

I was able to transition to ioredis fairly quickly, but that was stressful - and the library introduced its own challenges.

I do appreciate that sentinel support is non-trivial to implement.

REQUEST: Please make clear under your README limitations that sentinel is NOT supported, so developers can know this before
an unpleasant surprise at end of project.

[leibale]

@ChrisMoth I'm sorry you had to go through all of that. We are working on adding support for sentinel (see #302), hopefully I'll have good news soon... :)