document pre-defined secret generators

There's a handful of secret generators already defined in
`agenix-rekey.nix`.  This brings them into awareness via some README
updates.  I did a cursory read on each of the various commands used and
believe most are accurate if not very close descriptions of what they
do, but please scrutinize this as some of them (like `dhparams`) I am
not very familiar with.

README.md

@@ -229,6 +229,21 @@ the example below would generate a random 6 word passphrase using the
 }
 ```
 
+### Pre-defined generators
+
+- `alnum` - Generates a long (48 character) alphanumeric password.
+- `base64` - Generates a long (32 character) base64 encoded password.
+- `hex` - Generates a long (24 character) hexadecimal encoded password.
+- `passphrase` - Generates a six word, space separated passphrase.
+- `dhparams` - Generates Diffie–Hellman parameters which can be used for
+  perfect forward security.  See
+  [Diffie-Hellman_parameters](https://wiki.openssl.org/index.php/Diffie-Hellman_parameters)
+  for details.
+- `ssh-ed25519` - Generates a [ED-25519](https://en.wikipedia.org/wiki/EdDSA)
+  SSH key pair using the current hostname.
+
+### Custom generators
+
 You can also define your own generators, either by creating an entry in `age.generators`
 to make a reusable generator like `"passphrase"` above, or directly by setting
 `age.secrets.<name>.generator` to a generator definition.