Provide example for SSH flux secret

The examples assume the user is using HTTPS basic auth for their bootstrap git, which may not be a valid assumption. In such case, the command to create the private-git secret would be

kubectl create secret generic private-git --from-file=known_hosts=~/.ssh/known_hosts --from-file=identity=~/.ssh/<private key file> -n bigbang

As well, some older git providers (e.g. AWS CodeCommit) have outdated implementations, and require

patchesStrategicMerge:
- |-
  apiVersion: source.toolkit.fluxcd.io/v1beta2
  kind: GitRepository
  metadata:
    name: bigbang
  spec:
    gitImplementation: libgit2

in bigbang.yaml, and without which, will generate odd error messages from flux.

Please consider adding information like such to the examples.