Update bootstrap github docs to list permissions

Document the required permissions for bootstrapping pre-existing repositories.

I'm not sure if this is the correct place to document this as it feels to detailed for CLI output
but (as I understand it) the output of this is then passed to the website automatically and, not
knowing this took a while to debug the problems I was having.

Signed-off-by: Sam Cook <sam.cook@weave.works>

cmd/flux/bootstrap_github.go

@@ -38,11 +38,12 @@ import (
 var bootstrapGitHubCmd = &cobra.Command{
 	Use:   "github",
 	Short: "Bootstrap toolkit components in a GitHub repository",
-	Long: `The bootstrap github command creates the GitHub repository if it doesn't exists and
+	Long: `The bootstrap github command creates the GitHub repository if it doesn't exist and
 commits the toolkit components manifests to the main branch.
 Then it configures the target cluster to synchronize with the repository.
 If the toolkit components are present on the cluster,
-the bootstrap command will perform an upgrade if needed.`,
+the bootstrap command will perform an upgrade if needed.
+Admin permissions are needed on pre-existing repositories.`,
 	Example: `  # Create a GitHub personal access token and export it as an env var
   export GITHUB_TOKEN=<my-token>