Update snap.md

Author: chennes

technical/snap.md

@@ -10,9 +10,21 @@ FreeCAD is provided as a Snap package in multiple versions:
   * edge -- built daily from master
   * freecad-realthunder -- a convenience build of the LinkStage3 branch
 
-Each of these is represented by a branch at https://github.com/FreeCAD/FreeCAD-snap -- to make changes, switch to the appropriate branch.
+Each of these is represented by a branch at https://github.com/FreeCAD/FreeCAD-snap -- to make changes,edit [the snapcraft.yaml file](https://github.com/FreeCAD/FreeCAD-snap/blob/master/snap/snapcraft.yaml) on the branch you want to update.
 
-# Important links
+GitHub actions are used to do the actual building of each of these branches.
+
+## Routine maintenance
+
+The most common thing to break is the KDE Neon extension, which periodically needs to have its version number updated:
+if the snap-building logs contain errors about being unable to locate neon, check for the correct version number at
+https://snapcraft.io/docs/kde-neon-extension
+
+Snaps are built based on specific "cores" -- the base operating system whose libraries they use. From time to time
+we have to update to a new core. The current builds use "core20", or Ubuntu 20.04 LTS. Updating the core will likely
+require updating many of the other version numbers in the snap YAML file.
+
+## Important links
 
   * https://snapcraft.io/freecad
   * https://ubuntu.com/tutorials/create-your-first-snap