Clarify README with minor updates

Checklist

• I'm using the latest version of the extension (Run glab --version)
  • Extension version: Put your extension version here
• Operating System and version: _Put your version here

Summary

Steps to reproduce

What is the current bug behavior?

1. README Configuration section says the global configuration file is available at ~/.config/glab-cli, which is incomplete/incorrect.
2. README Configuration section says configuration can be set for "the current directory", which is not correct.
3. README Environment variables section does not include "GLAB_CONFIG_DIR".

What is the expected correct behavior?

1. README Configuration section should say the global configuration file is available at ~/.config/glab-cli/config.yml.
2. README Configuration section should say that configuration can be set for the current repo instead of current directory.
3. README Environment variables section should include "GLAB_CONFIG_DIR".

Relevant logs and/or screenshots

n/a

Possible fixes

1. https://gitlab.com/gitlab-org/cli/-/blob/main/README.md?plain=1#L152
2. https://gitlab.com/gitlab-org/cli/-/blob/main/README.md?plain=1#L154
3. https://gitlab.com/gitlab-org/cli/-/blob/main/README.md?plain=1#L160