Create snap.md

Stub of Snap-creation and maintenance documentation

Author: chennes

technical/snap.md

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+# Maintaining the Snap package
+
+FreeCAD is provided as a Snap package in multiple versions:
+  * stable -- built from the latest full release
+  * beta -- built weekly from master
+  * 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.
+
+# Important links
+
+  * https://snapcraft.io/freecad
+  * https://ubuntu.com/tutorials/create-your-first-snap