Perl6::TypeGraph is already outside the repo

And in the ecosystem, tested and documented. Good job
@antoniogamiz. Closes #2573

CONTRIBUTING.md

@@ -246,13 +246,19 @@ downloading the latest Rakudo Star release from
 installed Rakudo Star package, it should already be there. Feel free to
 use any other module installer for the modules needed (see below).
 
-#### Pod::To::HTML
+#### Building the documentation
 
 The program that builds the HTML version of the documentation
 (`htmlify.p6`) uses `Pod::To::HTML` to convert Pod 6 structures into HTML.
-You'll also need `Pod::To::BigPage`. Install these modules like so:
+You'll also need `Pod::To::BigPage` and `Perl6::TypeGraph`. Install these modules like so:
 
-    $ zef install Pod::To::HTML Pod::To::BigPage
+    $ zef install Pod::To::HTML Pod::To::BigPage Perl6::TypeGraph
+
+although these are included in the `META.json` file so with
+
+    $ zef install --deps-only .
+
+you're good to go.
 
 #### Mojolicious / Web Server