rustlings/exercises/README.md

Exercise to Book Chapter mapping

Exercise	Book Chapter
variables	§3.1
functions	§3.3
if	§3.5
primitive_types	§3.2, §4.3
vecs	§8.1
move_semantics	§4.1, §4.2
structs	§5.1, §5.3
enums	§6, §18.3
strings	§8.2
modules	§7
hashmaps	§8.3
options	§10.1
error_handling	§9
generics	§10
traits	§10.2
tests	§11.1
lifetimes	§10.3
standard_library_types	§13.2, §15.1, §16.3
threads	§16.1, §16.2, §16.3
macros	§19.6
clippy	n/a
conversions	n/a