Link the PGXN distribution from README and INSTALL

INSTALL

@@ -27,6 +27,14 @@ links against a shared-library Ruby (MRI).
 2. Build and install
 ---------------------
 
+PL/Ruby is on PGXN (https://pgxn.org/dist/plruby/), so with the PGXN client
+(https://pgxn.github.io/pgxnclient/) the whole build is:
+
+    pgxn install plruby
+
+That downloads the distribution and runs the same PGXS build described here.
+From a checkout, or to pass the overrides below:
+
     make
     sudo make install
 
@@ -42,6 +50,10 @@ Override them if needed:
 
     CREATE EXTENSION plruby;
 
+The PGXN client runs the same statement with:
+
+    pgxn load -d mydb plruby
+
 PL/Ruby is an untrusted language, so the extension is superuser-only to install
 and only superusers can create PL/Ruby functions.  See the SECURITY section of
 README for why.
@@ -72,6 +84,10 @@ and TRANSFORM FOR TYPE ltree (Ruby Array of label Strings):
 All three accept the same PG_CONFIG/RUBY overrides and have their own
 "make installcheck".
 
+"pgxn install plruby" builds only the top-level extension.  To add a transform
+on top of it, unpack the distribution ("pgxn download plruby") or use a
+checkout, and build the subdirectory as above.
+
 6. Packaging for PGXN (maintainers)
 -----------------------------------
 
@@ -83,8 +99,12 @@ archive from a committed tree with:
 
 That writes plruby-<version>.zip, taking the version from META.json, and the
 archive expands into a single plruby-<version>/ directory, which is the shape
-PGXN Manager expects at https://manager.pgxn.org/upload.
+PGXN Manager expects.
+
+Upload it at https://manager.pgxn.org/upload.  A new release shows up
+on master.pgxn.org right away; pgxn.org and api.pgxn.org serve it after their
+next mirror sync, so a 404 there for a short while is expected.
 
 When cutting a release, keep the "version" field in META.json in step with
-plruby.control and the CHANGELOG, and the "provides" versions in step with the
-per-extension .control files.
+plruby.control and the CHANGELOG, the "provides" versions in step with the
+per-extension .control files, and the PGXN badge version in README.

README.md

@@ -10,6 +10,7 @@
 [![Ruby](https://img.shields.io/badge/Ruby-3.x-CC342D?logo=ruby&logoColor=white)](https://www.ruby-lang.org/)
 [![Tests](https://img.shields.io/badge/tests-44_passing-brightgreen)](sql/)
 [![License](https://img.shields.io/badge/license-MIT-blue)](LICENSE)
+[![PGXN](https://img.shields.io/badge/PGXN-plruby_2.5.0-6d4c9f)](https://pgxn.org/dist/plruby/)
 
 </div>
 
@@ -104,17 +105,30 @@ $$;
 
 ## Installation
 
+From [PGXN](https://pgxn.org/dist/plruby/), with the
+[PGXN client](https://pgxn.github.io/pgxnclient/):
+
+```sh
+pgxn install plruby
+```
+
+Or from a checkout:
+
 ```sh
 make
 sudo make install
 ```
 
-Then, in a database:
+Either way builds from source, so PostgreSQL's server development files and a
+shared-library Ruby have to be present first (see
+[Requirements](#requirements)). Then, in a database:
 
 ```sql
 CREATE EXTENSION plruby;
 ```
 
+`CREATE EXTENSION` requires a superuser, because PL/Ruby is untrusted.
+
 See [**INSTALL**](INSTALL) for details, and run the regression suite with
 `make installcheck`.
 
@@ -137,6 +151,7 @@ to roles you would trust with the server's OS account.
 - [Performance benchmarks](doc/benchmark.md)
 - [Installation](INSTALL)
 - [Changelog](CHANGELOG.md)
+- [PGXN distribution](https://pgxn.org/dist/plruby/)
 - [Feature comparison: PL/Ruby vs PL/php vs PL/Perl vs PL/Tcl](doc/comparison.md)
 - [PL/Ruby vs PL/Perl](doc/plperl-comparison.md)
 - [PL/Ruby vs PL/Tcl](doc/pltcl-comparison.md)