Added a surprising behavior section on readdir.
Added a surprising behavior section on readdir.

For #18.

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