[epiphany/mcatanzaro/style] Add code style rule regarding comparisons

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 562f615c2c70c706f877fb8fefbe36a75dd974af
Author: Michael Catanzaro <mcatanzaro igalia com>
Date:   Tue Jan 1 16:39:34 2019 -0600

    Add code style rule regarding comparisons
    
    Epiphany code is currently inconsistent as to whether we should
    explicitly compare pointers against NULL. Very recently, I started
    writing explicit comparisons everywhere, after learning that this is now
    encouraged by the GLib maintainers. However, I've started having second
    thoughts. It's verbose, for one. Also, the C++ community is working on
    style guidelines to discourage this:
    
    http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#es87-dont-add-redundant--or--to-conditions
    
    They make sense to me. We already have a similar rule in WebKit, except
    WebKit also enforces this for integer comparisons against zero. I don't
    like that. Let's follow the C++ core guidelines advise instead.

 HACKING.md | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/HACKING.md b/HACKING.md
index 2ca51879c..2e287bcae 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -41,6 +41,10 @@ need to stick to in order to get your patch accepted:
  * There's no space between a type cast and the variable name:  Right:
    `(int *)foo`. Wrong: `(int*) foo`.
 
+ * Avoid explicit comparisons against TRUE, FALSE, and NULL. Right:
+   `if (!condition)`, `if (!pointer)`, `if (integer == 0)`. Wrong:
+   `if (condition == FALSE)`, `if (pointer == NULL)`, `if (!integer)`.
+
 # Code Structure
 
 ## Layering