add prefix and date to temporary folder

This makes identifying the latest version easier when daleing with backups.

README.md

@@ -403,7 +403,7 @@ An entropy pool value greater than 2000 is sufficient.
 Create a temporary directory which will be cleared on [reboot](https://en.wikipedia.org/wiki/Tmpfs) and set it as the GnuPG directory:
 
 ```console
-$ export GNUPGHOME=$(mktemp -d)
+$ export GNUPGHOME=$(mktemp -d -t gnupg_$(date +%Y%m%d%H%M)_XXX)
 ```
 
 Otherwise, to preserve the working environment, set the GnuPG directory to your home folder:
@@ -1800,7 +1800,7 @@ $ sudo mount /dev/mapper/secret /mnt/encrypted-storage
 Import the master key and configuration to a temporary working directory:
 
 ```console
-$ export GNUPGHOME=$(mktemp -d)
+$ export GNUPGHOME=$(mktemp -d -t gnupg_$(date +%Y%m%d%H%M)_XXX)
 
 $ gpg --import /mnt/encrypted-storage/tmp.XXX/mastersub.key