Add a section on troubleshooting [skip ci]

[peey]

Q	A
License	MIT
Doc PR	yes

Add a troubleshooting section to CONTRIBUTING.md for common gotchas while developing on babel

[codecov]

Codecov Report

Merging #5788 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5788      +/-   ##
==========================================
- Coverage   84.79%   84.76%   -0.04%     
==========================================
  Files         204      204              
  Lines        9607     9607              
  Branches     2701     2701              
==========================================
- Hits         8146     8143       -3     
- Misses        977      980       +3     
  Partials      484      484

Impacted Files	Coverage Δ
packages/babel-helper-call-delegate/src/index.js	64% <0%> (-4%)	⬇️
packages/babel-traverse/src/path/modification.js	73.78% <0%> (-0.98%)	⬇️
packages/babel-traverse/src/path/context.js	85.34% <0%> (-0.87%)	⬇️
...bel-plugin-transform-es2015-classes/src/vanilla.js	90.17% <0%> (ø)	⬆️

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[hzoo]

make bootstrap

[hzoo]

This is great, really appreciated!

[hzoo]

Oh, merged to master

[peey]

Oh, I accidentally made it to master. Maybe it's a github UI default. I cherry-picked all 3 commits and opened a new PR : #5790

Please merge that too @hzoo