Add ShaderGraph repository as a submodule

.gitmodules

 	path = PostProcessing
 	url = https://github.com/Unity-Technologies/PostProcessing
 	branch = v2
+[submodule "ShaderGraph"]
+	path = ShaderGraph
+	url = git@github.com:Unity-Technologies/ShaderGraph.git

ShaderGraph.meta

+fileFormatVersion: 2
+guid: c4d536ecfc4cf3c4999d7a81e2d718e6
+folderAsset: yes
+DefaultImporter:
+  externalObjects: {}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: