Add a README for Valgrind binaries.

README

Index: README
===================================================================
--- README	(revision 0)
+++ README	(revision 0)
@@ -0,0 +1,11 @@
+This directory contains pre-built valgrind binaries which can be used with
+NaCl. They can be updated from the ThreadSanitizer buildbot:
+    wget http://build.chromium.org/p/client.tsan/binaries/memcheck-r${REV}-amd64only-linux-self-contained.sh -O memcheck
+    wget http://build.chromium.org/p/client.tsan/binaries/tsan-r${REV}-amd64only-linux-self-contained.sh -O tsan
+
+Source code for these tools is available at:
+http://code.google.com/p/valgrind-variant
+http://code.google.com/p/data-race-test
+
+To find out the exact revision of the binaries, see "svn log".

[Mark Seaborn, 2011/04/13 06:26:48]

OK, if I do that, I find:
I don't understand what "@62+68+71" means.  Does it mean r62 with r68 and r71
cherry-picked?  (That seems unlikely if it is build automatically by a
buildbot.)

It's not immediately obvious that tsan@2519 refers to the directory
http://data-race-test.googlecode.com/svn/trunk/tsan/.  It wouldn't hurt to spell
this out.

[Evgeniy Stepanov, 2011/04/13 12:48:20]

Yes, the current binary was built manually with a couple of cherry-picked
changes. It should not happen in the future. Newest binaries from the buildbot
should be fine.
Yes, ideally commit messages should look like "Update memcheck and tsan binaries
to http://data-race-test.googlecode.com/svn/trunk@rXXXX." Valgrind revision is
pinned in third_party/update_valgrind.sh script.

Do you want me to fix the comment by changing SVN history (not sure if that's
allowed in NaCl)?

I plan to work on NaCl/DSO support in Valgrind and TSan in Q2. I'll update the
binaries with a proper comment then.

+