Clarify comment against github.repositories setting

[mungojam]

If we don't change the default then I'm not clear if that means all repositories or none.

It would be good to add to the comment to make it obvious.

[Tallion-07]

i have finally got it working! i was getting same problem as issue #43
it definitely required

1. ssh key setup in git for windows and entering ssh key in github user settings

in user settings it required

1. "github.host" as default
2. "github.username": "" example "chrmarti"
3. "github.repositories": "<username/repository-name"
   example "microsoft/vscode-github-issues-prs"

this didn't run straight "out of the box" on defaults and it took me ages to work out how to get it going
I had also setup the ssh keys first, and had nothing in user settings so it ran "out of the box" and nothing worked.
it wasn't until i put the settings as outlined above that it actually worked.
I think this is definitely needs to be documented in the README.md
I spent stupid amounts of time trying to get this working, it's all blindingly simple once you work it out, but for those that are still learning (like myself)

I didn't know in point 3. above if you needed

1. "https://github.com/microsoft/vscode-github-issues-prs"
2. "microsoft/vscode-github-issues-prs"
3. "vscode-github-issues-prs"

and if you don't put in "github.repositories": "<username/repository-name" it tells you that "@ has no issues"

once working it does display all issues that are both under milestones and also those that are not.