docs: add CLI help

docs/cmd/tk_export_source_git.md

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+## tk export source git
+
+Export git sources in YAML format
+
+### Synopsis
+
+Export git sources in YAML format
+
+```
+tk export source git [name] [flags]
+```
+
+### Examples
+
+```
+  # Export all git sources
+  export source git --all > sources.yaml
+
+  # Export a git source including the SSH keys or basic auth credentials
+  export source git my-private-repo --with-credentials > source.yaml
+
+```
+
+### Options
+
+```
+  -h, --help   help for git
+```
+
+### Options inherited from parent commands
+
+```
+      --all                  select all resources
+      --components strings   list of components, accepts comma-separated values (default [source-controller,kustomize-controller])
+      --kubeconfig string    path to the kubeconfig file (default "~/.kube/config")
+      --namespace string     the namespace scope for this operation (default "gitops-system")
+      --timeout duration     timeout for this operation (default 5m0s)
+      --verbose              print generated objects
+      --with-credentials     include credential secrets
+```
+
+### SEE ALSO
+
+* [tk export source](tk_export_source.md)	 - Export source commands
+
+###### Auto generated by spf13/cobra on 30-Apr-2020