Add snapcraft generated files to the Git tracking ignore rules

This patches uses the following gitignore syntax so that only entries in the root folder is ignored, it is suggested to apply it to existing rules as well.

```
A leading slash matches the beginning of the pathname. For
example, "/*.c" matches "cat-file.c" but not
"mozilla-sha1/sha1.c".
```

Refer-to: gitignore(5) manpage
Signed-off-by: 林博仁(Buo-Ren Lin) <Buo.Ren.Lin@gmail.com>

.gitignore

@@ -13,3 +13,10 @@ cabal-dev
 cabal.sandbox.config
 cabal.config
 .stack-work
+
+### Snap ###
+/snap/.snapcraft/
+/stage/
+/parts/
+/prime/
+*.snap