Add specific setup instructions for Vue users

[a-toms]

• Added `.default to allow Vue users to import turndown without failure. This is from https://stackoverflow.com/questions/55621010/using-turndown-in-vue-typeerror-turndownservice-is-not-a-constructor

[srbala]

LGTM

[domchristie]

I think this might be a more general issue with ES6 imports rather than a Vue-specific problem. Perhaps we need to update package.json?

[srbala]

If update to package.json solves the problem, that would be nice. This is PR is documentation update, anyone facing similar issue can resolve from it.

[pavelhoral]

This feels incorrect - the issue in the linked SO question seems to be with RequireJS and not at all connected with VueJS. So maybe you were not using the correct JS file (lib/turndown.browser.umd.js)?

For usage with RequireJS, UMD versions are located in lib/turndown.umd.js (for Node.js) and lib/turndown.browser.umd.js for browser usage.