doc: add note regarding file structure in src/README.md
Refs: https://github.com/nodejs/node/pull/34944#discussion_r480321239
Mostly adapted from @addaleax comment (I hope you don't mind) on the files structure since this information seems to be missing from our guides and it would be helpful for people unfamiliar/new with C++.
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines
/cc @nodejs/documentation @trott