Document need for editorConfig.highlightTheme

[jason0x43]

When using Snapguidist with Styleguidist 6.2.5, the styleguide config must explicitly set editorConfig.highlightTheme or Snapguidist will cause rendering errors when a snapshot test fails.

TypeError: Cannot read property 'replace' of undefined
    in CodeMirror (created by Code)
    in div (created by Code)
    in div (created by Code)
    in Code (created by Test)
    in div (created by Test)
    in div (created by Test)
    in div (created by Test)
    in Test (created by SnapguidistPreview)
    in div (created by SnapguidistPreview)
    ...

[MicheleBertoli]

Thank you very much, @jason0x43.
Fixed in v3.1.1.