Make checks scripts work on macOS + update readme

[hanickadot]

Scripts needs GNU sed, which is not available on macOS by default, but can be easily install with homebrew. If script detects presence of GNU sed, it will create a function to pass attributes from sed command to GNU sed.

I also updated readme.rst accordingly so it mentions homebrew so we don't need to point our users to download packages from internet, but they can just use the package manager.

Also I updated readme.rst and made all links https.

[jensmaurer]

Looks mostly good.

[jensmaurer]

@tkoeppe , this looks like an improvement. (The commit message could use an overhaul, mentioning the http -> https change as well. Maybe even in a separate commit.)

[tkoeppe]

What is "Instead of downloading packages from the internet" referring to? Does brew not download anything from the internet? And is this brew install mactex gnu-sed a complete replacement for the tlmgr installation?

[hanickadot]

Brew validates the packages against known checksums. It's against general recommendation "thou shall not download and install stuff random from internet". Brew also manages updating it and also makes easy to uninstall, without depending on package specific way how to remove stuff.

Yes, brew install mactex is a complete replacement for the tlmgr installation.

[hanickadot]

You can also install BasicTeX using brew and then use tlmgr, but it felt really convoluted and I felt I would end up describing all the combinations. My intention with this PR is just to provide quickest and most straightforwards way how to start with this repo, hence the auto config of gsed instead of asking users to change their whole environment and probably introduce some future problems.