Add readme and code mentions of values mutability

[paskal]

Add readme and code mentions of values mutability, fix #18.

I think we are free to release a new version after that. My proposition about version changelog:

- #11 introduce `LoadingCache.Close` function for proper LRU and Redis cache goroutines teardown
- #23 mention values mutability for LRU and Expirable cache in Readme, add code example for it
- #15 switch Expirable cache from `patrickmn/go-cache` to the internal implementation
- #11 stricter lint settings, bump dependencies
- #11 start running examples during tests
- #11 make tests work offline (had a dependency on internet connection)
- #11 #16 properly teardown goroutines to prevent their leakage in tests