Add note about cache version incompatibility to NWIS Client docs

[jarq6c]

Owing to #192 , it may behoove us to add a little note to the docs recommending against attempting to re-use caches after upgrading client versions.

[jarq6c]

Thanks @hellkite500 for the suggestion.

it might be worth a quick note in the documentation to clear those caches manually when upgrading versions (or try to detect the cache version and warn/auto regenerate???)

[aaraney]

Yeah, I agree. I think this would be a helpful addition to what could be an FAQ section in the nwiw_client docs. Having thought about it, I am hesitant to add a logic into the code base to check for this case, so I think docs are the way to go. Ill try to remember to open a PR to get this added in sometime this week.