Add section to easily migrate an entire folder of code into "Migrate from BuckleScript/Reason" page

[MoOx]

Because I think it's useful (and safe as soon as you have versioned codebase.

[ryyppy]

might be even worth mentioning to check in the compiled JS files first (compiled from the original .re files) before doing the conversion, bc checking the JS diffs is more reliable than just checking the new ReScript code.

Or to say it differently: if your compiled output doesn't change when compiling the files with the new syntax, you can be 100% sure your conversion didn't break any code

[MoOx]

That's a very good idea. I just added some instructions to do this comparison, tell me if that 's ok!

[harigopal]

This is nice! I'd just finished converting pupilfirst's ReasonML to ReScript and I'd written a small Ruby script to help automate the conversion.

One thing that could possibly be included in the total conversion snippet - for it to match .rei files to convert them to .resi.

[MoOx]

@harigopal not sure that's very necessary, a simple copy/paste with a 2 characters changes is acceptable I guess :D It's a "complicated" already line of code!

[ryyppy]

Thanks for your contribution!
I think i will merge this for now, but I will probably rephrase a few things later, if that is okay

[MoOx]

Feel free to rephrase :)