docs: add info about custom odk collect to advanced faq

docs/about/faq.md

@@ -391,3 +391,32 @@ Yes!
 - However, note that if there is no activity on a locked task
   after a period of time, the task may auto-unlock, allowing
   other mappers to lock and contribute.
+
+### What is the custom ODK Collect app?
+
+- ODK Collect is an excellent mobile application.
+- We need a slightly modified functionality of the app, so have created
+  a custom version for you to use with FMTM.
+- We maintain a good relationship with the ODK developers and would like
+  to get this functionality into the map app (meaning no custom app is required).
+- However, this will take time!
+
+### Why do I need the custom ODK Collect app?
+
+- Ideally when we open ODK Collect, we want our feature / building to be
+  selected already, avoiding the need to open the map and select it manually.
+- We created the custom ODK Collect for exactly this purpose!
+- It also opens the possibility of more advanced map capabilities in FMTM,
+  such as navigation to your chosen feature and directions.
+- Hopefully it provides a more seamless user experience of FMTM, that we
+  can't provide with the default Collect.
+
+### How do I install the custom ODK Collect app?
+
+- First, open the sidebar on FMTM and click the button to download the
+  custom ODK Collect `.apk` file.
+- If you don't have ODK Collect installed already, then you should be able to
+  click this file and install as normal.
+- If you already have ODK Collect installed, then you **may** have to uninstall
+  this version first, prior to installing the custom version. This is to avoid
+  version conflicts.