[gjs: 4/5] docs: Add instructions for installing mozjs78 with package manager

[Philip Chimento <pchimento src gnome org>]

commit 392f36a5fd3446f1401b516c00f12fcef082ba2d
Author: Philip Chimento <philip chimento gmail com>
Date:   Sat Jul 24 13:41:04 2021 -0700

    docs: Add instructions for installing mozjs78 with package manager
    
    And a bit more explanation of why you should build your own copy if you
    are writing any C++ code.

 doc/Hacking.md | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)
---
diff --git a/doc/Hacking.md b/doc/Hacking.md
index d5648cf8..ba562e0d 100644
--- a/doc/Hacking.md
+++ b/doc/Hacking.md
@@ -65,9 +65,25 @@ These debugging features reduce performance by quite a lot, but they
 will help catch mistakes in the API that could otherwise go unnoticed
 and cause crashes in gnome-shell later on.
 
-If you aren't writing any C++ code, then you don't need to build it
-yourself. Install SpiderMonkey using your system's package manager
-instead.
+If you aren't writing any C++ code, and your system provides it (for
+example, Fedora 33 or Ubuntu 20.10 and later versions), then you don't
+need to build it yourself.
+Install SpiderMonkey using your system's package manager instead:
+
+<details>
+    <summary>Ubuntu</summary>
+    <code>sudo apt-get install libmozjs-78-dev</code>
+</details>
+
+<details>
+    <summary>Fedora</summary>
+    <code>sudo dnf install mozjs78-devel</code>
+</details>
+
+If you _are_ writing C++ code, then please build SpiderMonkey yourself
+with the debugging features enabled.
+This can save you time later when you submit your merge request, because
+the code will be checked using the debugging features.
 
 To build SpiderMonkey, follow the instructions on [this 
page](https://github.com/mozilla-spidermonkey/spidermonkey-embedding-examples/blob/esr78/docs/Building%20SpiderMonkey.md)
 to download the source code and build the library.
 If you are using `-Dprefix` to build GJS into a different path, then