Mention security issue when recommending the user considers HashMaps

[NorfairKing]

If you don't care about ordering, consider using Data.HashMap.Lazy from the unordered-containers package instead.

Should say

If you don't care about ordering, and don't handle untrusted keys, consider using Data.HashMap.Lazy from the unordered-containers package instead.

[meooow25]

Seems reasonable. Would you like to open a PR for it?