Add Troubleshooting to Github Import

Description

• Include troubleshooting section, with some issues users have seen.

Proposal

Troubleshooting

• Syntax is very specific. Remove spaces within argument block and prior to brackets

# Success
sudo gitlab-rake import:github[access_token,root,foo/bar]

# Fail
sudo gitlab-rake import:github[access_token, root, foo/bar]
rake aborted!
Don't know how to build task 'import:github[access_token, root, foo/bar,' (see --tasks)

• Some shells can interpret the open/close brackets ([]) separately (ex: zsh). You may need to escape the brackets or switch shells

zsh: no matches found: import:github[token,root,foo/project,group/repository]

Links / references

https://docs.gitlab.com/ce/administration/raketasks/github_import.html

Internal Zendesk: https://gitlab.zendesk.com/agent/tickets/91464