[epiphany/mcatanzaro/valgrind-instructions] Add instructions on how to use valgrind

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 4bc9a7e3973086d43f37f9e5b02301327703995f
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Tue Nov 26 22:41:54 2019 +0000

    Add instructions on how to use valgrind

 HACKING.md | 9 +++++++++
 1 file changed, 9 insertions(+)
---
diff --git a/HACKING.md b/HACKING.md
index 7a04f6e0c..f8e7c0f61 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -208,3 +208,12 @@ filename. E.g. `export EPHY_PROFILE_MODULES=ephy-window.c:ephy-autocompletion.c`
 The special profiling module `all` enables all profiling modules.
 
 Use `START_PROFILER STOP_PROFILER` macros to profile pieces of code.
+
+## valgrind
+
+To use valgrind, you must use environment variables to disable WebKit's sandbox
+and JavaScriptCore's Gigacage:
+
+```
+WEBKIT_FORCE_SANDBOX=0 GIGACAGE_ENABLED=0 epiphany
+```