add redis cache hint to docs

[linroex]

If use Redis as cache server, Flask-Cache will say driver not found, user need manual install python-redis library, so I add hint text into docs

Ref.
Flask-cache redis cache failed

[coveralls]

Coverage remained the same at 81.023% when pulling 886a160 on linroex:master into 299e31f on airbnb:master.

[xrmx]

Strangely enough it's not documented on flask-cache, cannot hurt to have it there though :)

[linroex]

I think it is important, because flask-cache say redis is builtin, but if fact you need install python-redis, like Memcached, many people will feel adoubt (ex. me)

[mistercrunch]

Thanks for the clarification. We use memcache but the 1M limits and workarounds are a bit annoying... Maybe we should go with Redis.

[rhunwicks]

Note that in order to use a Redis cache with Superset you need to pip install redis. There is actually another package called python_redis and if you do pip install python-redis you get the wrong one. The hint that is now in the docs has the correct link, but might confuse people because it says "please install
 python-redis". It would be better if it said "please install the redis Python package." or similar.

[xrmx]

@rhunwicks feel free to open a PR