Add comment about `devtools_page` permissions request to manifest.json

[peterhartree]

If you include devtools_page in your extension manifest, the extension will show the following permissions warning on install:

• Read and change all your data on all websites

I wasted quite some time trying to figure out why I was getting this warning—it's not obvious that the devtools_page thing would trigger it.

I suggest adding a comment about this, or else removing the line completely.