Documentation on how to compile frontend code for building from source?

[kapily]

I'm trying to create a PR to modify some frontend code (.tsx) but not sure how I can build from source so I can test the entire change locally.
Could you add some documentation on how to modify the UI code locally and have it compile the javascript and then be able to run mercury?

Would make it easier to contribute, thanks!

[pplonski]

Thanks @kapily for pointing this, @apjanusz please add contributing to docs