Mention requirement of reasonable Ord instance

[josephcsible]

If a map's keys are Floats, Doubles, or some other type with an unreasonable Ord instance (not saying "unlawful" because Ord has no laws ☹️), then the guarantees provided by justified-containers don't hold. Example:

import Prelude hiding (lookup)
import Data.Map.Justified
import qualified Data.Map as M

test_table :: M.Map Float String
test_table = M.fromList [ (0, "foo"), (-1, "bar"), (0/0, "oops"), (-5, "baz"), (-6, "qux"), (-3, "quux") ]

main :: IO ()
main = withMap test_table $ \table -> do
  case member 0 table of
    Nothing -> putStrLn "Sorry, I couldn't prove that the key is present."
    Just key -> do
      putStrLn ("Found key: " ++ show key)
      putStrLn ("Value for key: " ++ lookup key table)
      inserting (0/0) "whoops" table $ \(newKey, upgrade, table') -> do
        putStrLn ("Value for key in updated map: " ++ lookup (upgrade key) table')

Found key: Key 0.0
Value for key: foo
Value for key in updated map: Main.hs: Data.Map.Justified has been subverted!
CallStack (from HasCallStack):
  error, called at src/Data/Map/Justified.hs:402:17 in justified-containers-0.3.0.0-QwQWf1ossWJ9BbDcxnzmi:Data.Map.Justified

There's obviously nothing justified-containers can really do about this, so I'd just like it to be mentioned in the documentation.

[Elvecent]

gdp package has Lawful constraint wrapper intended to mark "correct" instances. Of course, Double and Float aren't supposed to be Lawful Eq, and, therefore, Lawful Ord. But this is additional overhead, so...