Added a note on mmap durability for #21.
Added a note on mmap durability for #21.

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