MINOR: reg-tests: Add reg-tests/README file.

Add reg-tests/README file about how to compile and use varnishtest, and
how to produce patches to add regression testing files to HAProxy sources.

Also update CONTRIBUTING file to encourage the contributors to write
regression testing files.
diff --git a/CONTRIBUTING b/CONTRIBUTING
index b2c2b49..9e0d625 100644
--- a/CONTRIBUTING
+++ b/CONTRIBUTING
@@ -467,6 +467,12 @@
    harm when nobody is able to tell whether or not the patch needs to be
    backported or can be reverted in case of regression.
 
+   When fixing a bug which is reproducible, if possible, the contributors are
+   strongly encouraged to write a regression testing VTC file for varnishtest
+   to add to reg-tests directory. More information about varnishtest may be
+   found in README file of reg-tests directory and in doc/regression-testing.txt
+   file.
+
 12) Discuss on the mailing list
 
    When submitting changes, please always CC the mailing list address so that
@@ -615,6 +621,9 @@
 
   - tests     regression test files. No code is affected, no need to upgrade.
 
+  - reg-tests regression test files for varnishtest. No code is affected, no
+              need to upgrade.
+
   - init      initialization code, arguments parsing, etc...
 
   - config    configuration parser, mostly used when adding new config keywords