Add reference to trocla's README

the output of `trocla --help` is really not helpful. With the README
file at least we can see what other subcommands exist and how they're
used.

There was a URL defined for "Trocla" but it wasn't used, so I just
linked it up on the first appearance of the software name.

howto/puppet.md

@@ -366,10 +366,17 @@ in Puppet source code, for various reasons: it is hard to erase
 because code is stored in git, but also, ultimately, we want to
 publish that source code publicly.
 
-We use Trocla for this purpose, which generates
+We use [Trocla][] for this purpose, which generates
 random passwords and stores the hash or, if necessary, the clear-text
 in a YAML file.
 
+[Trocla]: https://github.com/duritong/trocla
+
+Trocla's man page is not very useful, but you can see a list of subcommands in
+the [project's README file][].
+
+[project's README file]: https://github.com/duritong/trocla
+
 With Trocla, each password is generated on the fly from a secure
 entropy source ([Ruby's SecureRandom module][]) and stored inside a
 state file (in `/var/lib/trocla/trocla_data.yml`, configured
@@ -382,7 +389,6 @@ it's been transmitted to the user or another password manager. This
 makes it possible to have Trocla not keep any secret at all.
 
 [Ruby's SecureRandom module]: https://ruby-doc.org/stdlib-1.9.3/libdoc/securerandom/rdoc/SecureRandom.html
-[Trocla]: https://github.com/duritong/trocla
 
 This piece of code will generate a [bcrypt][]-hashed password for the
 Grafana admin, for example: