README Outdated: Background environments file no longer exists and replaced by gentle.js

**Issue Description**
Under the `Customize your own simulation` header in the README, `#3`, it states that I should update the `data/firstmap.ts` file to change the background environment. Except that **file doesn't exist**.

It seems to have been replaced by a `gentle.js` during a `Merge pull request #177`.

**Expected Behavior**
I expected to see a `firstmap.ts` file in the `data` directory, but instead found a `gentle.js`.

**Optional Request**
I also was further confused by the fact that there are several different tilesets in `src/components/editor/tilesets` + `src/components/editor/map`.

I really appreciate AI Town V2's emphasis on making it easy to mod. I would love a deeper explanation on how to take advantage of the existing tilesets.

Thanks!