[epiphany] HACKING.md: Add note about developer mode for the web process extension

[Marge Bot <marge-bot src gnome org>]

commit f3f36e29b75b7e2fd29dbf4afba7ac898d630827
Author: vanadiae <vanadiae35 gmail com>
Date:   Sun Dec 26 15:42:18 2021 +0100

    HACKING.md: Add note about developer mode for the web process extension
    
    That's something I encountered, and that totally confused me when I was
    making changes to ephy.js. So add it to the HACKING.md in the proper
    section so that there's a chance the next person behind me that doesn't
    know the web process extension will find it :)
    
    Part-of: <https://gitlab.gnome.org/GNOME/epiphany/-/merge_requests/1052>

 HACKING.md | 7 +++++++
 1 file changed, 7 insertions(+)
---
diff --git a/HACKING.md b/HACKING.md
index 8a050769f..8f935fbf5 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -155,6 +155,13 @@ functions webkit_web_context_send_message_to_all_extensions(),
 webkit_web_view_send_message_to_page(), webkit_web_extension_send_message_to_context(),
 and webkit_web_page_send_message_to_view().
 
+If you are making changes to the web process extension, you'll need to enable
+developer mode as described below so that Epiphany will look for the shared
+library in your build directory, instead of using the one from the installed location,
+which would correspond to your installed Epiphany's web process extension. So if
+changes you make to the web process extension (or its javascript files) aren't
+picked up, it means you didn't enable developer mode.
+
 Epiphany uses script message handlers as an additional form of IPC. This allows
 the web extension to send a `WebKitJavascriptResult` to the UI process, which
 is received in `EphyEmbedShell`.