Add note about using multiple Redis versions

[abhijoshi2k]

Added new option to connect using connection string.

redisUrl - A Redis connection string to be used for the default client connection. Ignored when the client option is provided. Refer Redis Client connection string format.

Closes #341

[wavded]

Thanks for contributing! Unfortunately, this approach causes an issue with users who don't use redis now being required to have that installed. This used to be the approach when that was the only clients but now you bring your own client.

The other issue, is this doesn't really solve all the compatibility issues with how Redis is actually used in this project. See #337 (comment) for more details.

[abhijoshi2k]

Thanks for response. I'm not suggesting this as permanent option. However, it can be a temporary fix so that users would be able to use the package even with node-redis v4. We can specify in documentation that the client option is only supported for v3 and redisUrl option will use node-redis v3 by default.
I think, that is how rate-limit-redis package can still be used with redisURL option even though the client option is affected.
After things get sorted with breaking changes of node-redis v4, this option can be removed.

[abhijoshi2k]

As per recent comment on related issue, I think the previous changes proposed in this PR will not be necessary. However, I have updated the documentation showing a note about usage with redis v4. If these changes are okay, feel free to merge this PR or suggest edits. It is also fine if these updates to documentation are not acceptable. In that case, close this PR.

[wavded]

Thx @abhijoshi2k !