Document PG_TEST_NOCLEAN in TAP test README

Commit 90627cf98 added support for retaining the data directory even on
successful tests, but failed to document the environment variable which
controls retention. This adds a small note to the TAP test README about
PG_TEST_NOCLEAN which when set skips removing the data directories from
successful tests.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/2B02C1B3-3F41-4E14-92B9-005D83623A0B@yesql.se

src/test/perl/README

@@ -23,6 +23,11 @@ tmp_check/log/ to get more info.  Files named 'regress_log_XXX' are log
 output from the perl test scripts themselves, and should be examined first.
 Other files are postmaster logs, and may be helpful as additional data.
 
+Data directories will also be left behind for analysis when a test fails;
+they are named according to the test filename.  But if the environment
+variable PG_TEST_NOCLEAN is set, data directories will be retained
+regardless of test status.
+
 
 Writing tests
 -------------