add docs on XML Notepad File Association.

docs/help/fileassociation.md

@@ -0,0 +1,40 @@
+## XML Notepad File Association
+
+See the following video that shows how to associate XML Notepad with various .xml file types:
+
+[Youtube Video](https://youtu.be/n-6sSUSlN34)
+
+The general approach that works is to do the following:
+
+1. Open the XML Notepad that you installed already.
+2. Select Help/Open Sample... and select "Hamlet.xml"
+3. You will see a long file path in the address bar like this:
+
+> C:\Program Files\WindowsApps\43906ChrisLovett.XmlNotepad_2.9.0.5_neutral__hndwmj480pefj\Application\Samples\Hamlet.xml
+
+If you open this path in Windows Explorer you can locate XmlNotepad.exe here:
+
+> C:\Program Files\WindowsApps\43906ChrisLovett.XmlNotepad_2.9.0.5_neutral__hndwmj480pefj\Application\XmlNotepad.exe
+
+Copy this path to the clipboard.
+
+Now use the start menu to find the "Default apps" settings dialog.
+
+Enter the file type at the top (e.g. type in ".xsd" or ".xslt"),
+then click the button labelled `+ Choose a default`.
+
+Scroll to the bottom of this dialog and click `Choose an app on your PC`
+
+And paste in the path you coped to the clipboard earlier.
+Click `Open` then click `Set Default`.
+
+As soon as you do this the icon on the file type you associated  should change
+in Windows Explorer showing the XML Notepad icon and when you double click the
+file it will automatically open XML Notepad.
+
+Note that when you have done one extension, all the other extensions are much
+easier because XML Notepad will now show up in the list of possible apps to
+choose.
+
+The path you have registered here is version specific so you may have to repeat
+this process when you install new versions of XML Notepad.

docs/help/overview.md

@@ -39,3 +39,5 @@ The status bar at the bottom shows various messages indicating the state of the
 
 ### Resizers
 You can move the resizers to change the ratio of space allocated to each of the main three panels.
+
+See also [XML Notepad File Association](help/fileassociation.md).

docs/help/settings.md

@@ -40,4 +40,6 @@ and it will use the first one that it finds.
 o make it easier to pre-configure XML notepad across a bunch of machines, you can provide a
 customized `XmlNotepad.template.settings` template file next to `XmlNotepad.exe` and the first time
 a user launches XmlNotepad on a machine it will use this template for the initial default settings
-for that user, which is then copied to the `Roaming` location.
+for that user, which is then copied to the `Roaming` location.
+
+See also [XML Notepad File Association](help/fileassociation.md).

docs/install.md

@@ -41,4 +41,6 @@ winget install XmlNotepad
 
 WebView2 is an optional component that you can install from Microsoft. XML Notepad will use this component as the HTML rendering engine
 behind the [XSLT Output tab](help/xslt.md).  WebView2 provides a
-significant performance boost on large XSLT outputs.
+significant performance boost on large XSLT outputs.
+
+See [XML Notepad File Association](help/fileassociation.md).