Clarify that the result of `readQuery` should not be modified (#4815)
Clarify that the result of `readQuery` should not be modified (#4815)

The `readQuery` call may return an object that is shared with the current contents of cache, which can cause some weird behavior if you mutate the `readQuery` result.

See also:
https://github.com/apollographql/apollo-client/pull/4514
https://github.com/apollographql/apollo-client/pull/4543
apollographql/apollo-client,apollostack/apollo-client,apollostack/apollo-client,apollographql/apollo-client,apollostack/apollo-client