Add Sourcetree manifest (#339)

* Added Sourcetree manifest

* Updated architecture to x64

* Added silent switches.

* Moved version to line below.

* Deleted file.

* Re-added file.

Co-authored-by: Ollie Harker <oliver.harker@dyballassociates.co.uk>

manifests/Atlassian/Sourcetree/3.3.8.yaml

@@ -0,0 +1,17 @@
+Id: Atlassian.Sourcetree
+Name: Sourcetree
+Version: 3.3.8
+Publisher: Atlassian
+License: Copyright (c) Atlassian 2020
+AppMoniker: Sourcetree
+Tags: Source Control Client
+Description: Sourcetree is a free Git desktop client for developers on Windows. Say goodbye to the command line and use the full capabilities of Git through Sourcetree's beautifully simple interface (and stop being jealous of what your Mac friends are using).
+Homepage: https://www.sourcetreeapp.com/
+Installers:
+  - Arch: x64
+    Url: https://product-downloads.atlassian.com/software/sourcetree/windows/ga/SourceTreeSetup-3.3.8.exe
+    Sha256: 527468325C27B6AD055C4FC34092D980136340A84207E6B2DA5362465083E5C0
+    InstallerType: exe
+    Switches:
+      Silent: /silent
+      SilentWithProgress: /silent