Doc: add some doco about using the libpq_pipeline test module.

The README file here was barely a stub.  Try to make it useful.

Jelte Fennema, with some further work by me

Discussion: https://postgr.es/m/AM5PR83MB0178D3B31CA1B6EC4A8ECC42F7529@AM5PR83MB0178.EURPRD83.prod.outlook.com

src/test/modules/libpq_pipeline/README

@@ -1 +1,35 @@
 Test programs and libraries for libpq
+=====================================
+
+This module was developed to test libpq's "pipeline" mode, but it can
+be used for any libpq test that requires specialized C code.
+
+"make check" will run all the tests in the module against a temporary
+server installation.
+
+You can manually run a specific test by running:
+
+    ./libpq_pipeline <name of test> [ <connection string> ]
+
+This will not start a new server, but rather connect to the server
+specified by the connection string, or your default server if you
+leave that out.  To discover the available test names, run:
+
+    ./libpq_pipeline tests
+
+To add a new test to this module, you need to edit libpq_pipeline.c.
+Add a function to perform the test, and arrange for main() to call it
+when the name of your new test is passed to the program.  Don't forget
+to add the name of your test to the print_test_list() function, else
+the TAP test won't run it.
+
+If the order in which Postgres protocol messages are sent is deterministic
+in your test, you should arrange for the message sequence to be verified
+by the TAP test.  First generate a reference trace file, using a command
+like:
+
+   ./libpq_pipeline -t traces/mynewtest.trace mynewtest
+
+Then add your test's name to the list in the $cmptrace definition in the
+t/001_libpq_pipeline.pl file.  Run "make check" a few times to verify
+that the trace output actually is stable.