Added docs for NotImplementedFileSystem.
Added docs for NotImplementedFileSystem.

jacobsa/timeutil,GoogleCloudPlatform/gcsfuse,kahing/gcsfuse,GoogleCloudPlatform/gcsfuse,GoogleCloudPlatform/gcsfuse,BanzaiMan/gcsfuse,mbrukman/gcsfuse,BanzaiMan/gcsfuse,jacobsa/ratelimit