Doc: add note about how to run the pg_amcheck regression tests.

It's not immediately obvious what you have to do to get "make
installcheck" to work here, so document that along the same lines
as we've used elsewhere.

src/bin/pg_amcheck/README

+src/bin/pg_amcheck/README
+
+pg_amcheck is a command-line tool for running the amcheck extension.
+
+Running the regression tests
+============================
+
+NOTE: You must have given the --enable-tap-tests argument to configure.
+Also, to use "make installcheck", you must have built and installed
+contrib/amcheck and contrib/pageinspect in addition to the core code.
+
+Run
+    make check
+or
+    make installcheck
+You can use "make installcheck" if you previously did "make install".
+In that case, the code in the installation tree is tested.  With
+"make check", a temporary installation tree is built from the current
+sources and then tested.