Make it more clear how to specify zig version

[palsmo]

In the readme it's shown:
packages.<version>
When I tried doing the obvious step:

packages.0.13.0 -> fail
packages.<0.13.0> -> fail

I had to go and search in github for others, to find you are supposed to do:
packages."0.13.0"
For me who is newer to nix this was not obvious.
Can we make it more clear in the README.md:
packages."<version>" or something.
Thanks.

[Yappaholic]

It's more about how nix handles different package attributes, because version needs to be a string, like how nodePackages work (to install package you can use nodePackages."typescript").

Else you would have to create specific package for every version, the way nixpkgs do with zig_0_9, zig_0_12, etc.

Guess I'll update #56 to make this more clear.