Improve documentation on per-file-ignores

Mentions per-file-ignores as possible way to ignore specific rules.
Status Job ID Name Coverage
  Test
passed #415318064
docs

00:01:04

passed #415318065
dogfood

00:00:47

passed #415318062
linters

00:01:40

passed #415318063
pre-commit

00:01:15

passed #415318054
pypy2

00:02:27

passed #415318055
pypy3

00:02:10

passed #415318056
python2

00:00:59

failed #415318057
python34

00:01:00

passed #415318058
python35

00:00:56

passed #415318059
python36

00:00:56

passed #415318060
python37

00:00:54

passed #415318061
python38

00:00:52

 
  Build
skipped #415318066
build
 
Name Stage Failure
failed
python34 Test

Tip: See all example repositories: https://github.com/codecov?query=example
Support channels:
Email: [email protected]
IRC: #codecov
Gitter: https://gitter.im/codecov/support
Twitter: @codecov

ERROR: Job failed: exit code 1