Give possible values for memory_mb (#377)

I have to look this up every time I use this action. I think it's
helpful to list the possible values here?

Author: greg-finley

README.md

@@ -57,7 +57,7 @@ jobs:
 
 - `entry_point`: (Optional) Name of a function (as defined in source code) that will be executed. Defaults to the resource name suffix, if not specified.
 
-- `memory_mb`: (Optional) The amount of memory in MB available for a function. Defaults to '256' (256 MB). The value must be the number of megabytes _without_ the unit suffix.
+- `memory_mb`: (Optional) The amount of memory in MB available for a function. Defaults to '256' (256 MB). The value must be the number of megabytes _without_ the unit suffix. Possible values documented [here][memory].
 
 - `region`: (Optional) [Region](https://cloud.google.com/functions/docs/locations) in which the function should be deployed. Defaults to `us-central1`.
 
@@ -268,6 +268,7 @@ Credentials.
 
 [cloud-functions]: https://cloud.google.com/functions
 [runtimes]: https://cloud.google.com/sdk/gcloud/reference/functions/deploy#--runtime
+[memory]: https://cloud.google.com/sdk/gcloud/reference/functions/deploy#--memory
 [sm]: https://cloud.google.com/secret-manager
 [wif]: https://cloud.google.com/iam/docs/workload-identity-federation
 [sa]: https://cloud.google.com/iam/docs/creating-managing-service-accounts