Document memcache max key length

Signed-off-by: David Weitzman <dweitzman@pinterest.com>

README.md

@@ -567,6 +567,9 @@ To configure a Memcache instance use the following environment variables instead
 With memcache mode increments will happen asynchronously, so it's technically possible for
 a client to exceed quota briefly if multiple requests happen at exactly the same time.
 
+Note that Memcache has a max key length of 250 characters, so operations referencing very long
+descriptors will fail.
+
 # Contact
 
 * [envoy-announce](https://groups.google.com/forum/#!forum/envoy-announce): Low frequency mailing

src/memcached/cache_impl.go

@@ -9,6 +9,9 @@
 // limit the number of RPCs. First we call increment, then add if the increment
 // failed, then increment again if the add failed (which could happen if there was
 // a race to call "add").
+//
+// Note that max memcache key length is 250 characters. Attempting to get or increment
+// a longer key will return memcache.ErrMalformedKey
 
 package memcached