README.md: document the "named exercise" mode

[perillo]

By named exercise mode I mean the following steps:

name	description
1_test	Run 001_hello.zig without checking output
1_install	Copy 001_hello.zig to zig-cache/bin
1	Check 001_hello.zig only

These steps/command are not documented in the README.md file.

Documentation for Step n has been added recently as workaround for making ziglings work again, but in my next commit to the perillo:improve-build branch, I will restored the README.md file to the previous state.

[chrboesch]

These steps/command are not documented in the README.md file.

But you can see it when you run zig build -h, what the standard is.

[perillo]

And it is already documented in the Advanced section, that I originally missed.