Add "main entry in C" documentation section

[danielchasehooper]

While transitioning a C project to Zig, it tripped me up that Zig expected main() to be in Zig. Took a while for me to find the workaround. I felt the docs should mention it so others can avoid the same snag.

This is what it looks like:

[danielchasehooper]

This recommendation comes from Andrew. found here.

The example works with zig master on macos. Are you on windows or an older version of zig?

[rohlem]

The example works with zig master on macos.

It probably works on macOS because that uses libc as its system ABI, therefore including it by default iirc.
The full example linked does in fact call .linkLibC(), which is equivalent to specifying the -lc flag.

[danielchasehooper]

Ok, added -lc flag.

[gooncreeper]

Under Root Source File there is already a relevant TODO: if linking with libc you can use export fn main, you should put the section there and additionally add an example of a C main in Zig.

[danielchasehooper]

Under Root Source File ... you should put the section there

Since this information is specifically about mixed C/Zig code, I think it belongs in the section about integrating C and Zig. The fact that the solution involves the root source file is incidental.

additionally add an example of a C main in Zig.

I do not know how to do a C main in Zig. This PR is about the opposite problem.

Would someone with the power to merge mind chiming in? cc @andrewrk @alexrp