diff --git a/src/testdir/README.txt b/src/testdir/README.txt
index e9e9aeb..b8bc52f 100644
--- a/src/testdir/README.txt
+++ b/src/testdir/README.txt
@@ -41,6 +41,21 @@
   with "Skipped" so that it's clear this still needs work.  E.g.: throw
   "Skipped: Bug with <c-e> and popupmenu not fixed yet"
 
+- The following environment variables are recognized and can be set to
+  influence the behavior of the test suite (see runtest.vim for details)
+
+  - $TEST_MAY_FAIL=Test_channel_one    - ignore those failing tests
+  - $TEST_FILTER=Test_channel    - only run test that match this pattern
+  - $TEST_SKIP_PAT=Test_channel  - skip tests that match this pattern
+  - $TEST_NO_RETRY=yes           - do not try to re-run failing tests
+  You can also set them in Vim:
+    :let $TEST_MAY_FAIL = 'Test_channel_one'
+    :let $TEST_FILTER = '_set_mode'
+    :let $TEST_SKIP_PAT = 'Test_loop_forever'
+    :let $TEST_NO_RETRY = 'yes'
+  Use an empty string to revert, e.g.:
+    :let $TEST_FILTER = ''
+
 - See the start of runtest.vim for more help.
 
 
@@ -76,19 +91,24 @@
 test fails, then the test.log file contains the error messages.  If all the
 tests are successful, then this file will be an empty file.
 
-To run a single test function from a test script:
+- To run a single test function from a test script:
 
     $ ../vim -u NONE -S runtest.vim <test_file>.vim <function_name>
 
-To run all the tests:
+- To execute only specific test functions, add a second argument:
+
+	 $ ../vim -u NONE -S runtest.vim test_channel.vim open_delay
+
+
+- To run all the tests:
 
     $ make
 
-To run the test on MS-Windows using the MSVC nmake:
+- To run the test on MS-Windows using the MSVC nmake:
 
     > nmake -f Make_dos.mak
 
-To run the tests with GUI Vim:
+- To run the tests with GUI Vim:
 
     $ make GUI_FLAG=-g
 
@@ -96,7 +116,6 @@
 
     $ make VIMPROG=../gvim
 
-To cleanup the temporary files after running the tests:
+- To cleanup the temporary files after running the tests:
 
     $ make clean
-