Add info to README about layered requirements files

[jamescooke]

Hopefully fixes #398 🤞

Changelog-friendly one-liner: Add info to README about layered requirements files and -c flag

Contributor checklist

Not sure how to address these - sorry...

• Provided the tests for the changes.
• Requested a review from another contributor.
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md on release).
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[jamescooke]

This is broken because master is broken 😞

Latest PR that seems to have introduced the breakage is here: #864

Bump me when master is green again and I can merge the fixes.

[atugushev]

LGTM! Thanks for your contribution! Please, consider grammar comments as minor, for I'm not a native speaker.

[atugushev]

This is broken because master is broken 😞

Latest PR that seems to have introduced the breakage is here: #864

Looks like I've forgotten to mark pip19.3 as allowed to fail.

[codecov]

Codecov Report

Merging #905 into master will decrease coverage by 0.48%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #905      +/-   ##
==========================================
- Coverage   99.06%   98.57%   -0.49%     
==========================================
  Files          35       35              
  Lines        2247     2247              
  Branches      288      288              
==========================================
- Hits         2226     2215      -11     
- Misses         13       21       +8     
- Partials        8       11       +3

Impacted Files	Coverage Δ
piptools/repositories/pypi.py	91.21% <0%> (-3.91%)	⬇️
piptools/utils.py	97.95% <0%> (-2.05%)	⬇️

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[jamescooke]

@atugushev Thanks for all the suggestions - I've merged them all in.

Just waiting for the master fix now (#906) 👍

[atugushev]

@jamescooke

Thanks for addressing my review comments! Could you please also add a "Changelog-friendly one-liner" to the issue text?

[jamescooke]

@atugushev NP. Thanks for helping me on this PR 👍

I have added the one-liner to the description - hope it's suitable.

[atugushev]

Great! I'm going to merge this, despite failing CI, since the TOXENV=readme CI job is green.

[atugushev]

@jamescooke thank you so much for working on this! Appreciate it!