Add a description for installing GraphicsMagick or ImageMagick in docs/README.md (#4045)
Add a description for installing GraphicsMagick or ImageMagick in docs/README.md (#4045)

* Add a troubleshooting in README for an error about gm

* Solve linting

* Change dirname from local to anonymous in the example error message

* Add mentioning ImageMagick

mochajs/mocha,adamgruber/mocha,mochajs/mocha,outsideris/mocha,adamgruber/mocha,mochajs/mocha,adamgruber/mocha,outsideris/mocha