Datasync instructions need updated

[mnickels]

Just did a fresh setup via the CDK, and when I was walking through the manual steps under Option 2: DataSync and S3 section of the readme, there's a couple things missing that might've changed since the last time these steps were updated:

1. Under "Step 2: Create an EFS -> S3 DataSync Task", there's 2 additional fields that the form requires for the EFS location:
   1. Subnet - I think any of the subnets created during setup will work, I've tested with my minecraft-server-stack/Vpc/PublicSubnet1 and minecraft-server-stack/Vpc/IsolatedSubnet3 and they both work (as long as the next setting is correct)
   2. Security Group - this must be the one that starts with minecraft-server-stack-ServiceSecurityGroup, using the other SG created (minecraft-server-stack-FileSystemEfsSecurityGroup) or a default SG will not work OOTB
2. For the files included, you have to specify a / in front of the file names, which is weird but the form fails validation if any of the fields are missing it (personally, I just use the "Entire source location" option to avoid the hassle)