docs: add default keybinding maps available in the help

[makbari]

Description

Updated README.md file with the default keymapings available in :h gitlab.nvim.keybindings.

[harrisoncramer]

Hey this looks good to me and I'm happy to merge it in, but could you please change the target branch to develop and merge in latest? Then I'll merge it in. That's our regular release pattern.

[jakubbortlik]

Hi, the call is obviously Harrison's but I'd rather not add the keybindings to the README. It would be the third place to keep in sync with doc/gitlab.nvim.txt and lua/gitlab/state.lua. To me the two existing documents are already enough :smile:

I mean, I appreciate your effort @makbari, but in my view, the information is quite easy to find already.

[makbari]

@jakubbortlik Thanks for pointing out. I did not know about those files. Indeed, these files are more than enough. Maybe it is helpful for the users who just want to find the keybindings quickly.

For myself, i found the keybinding in the help doc in nvim. If this is not helping, let me know, I close the pr.