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>

1 files changed, 7 insertions, 0 deletions

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`.