chore(README.md): document "main" field

Author: Boshen

README.md

@@ -95,6 +95,20 @@ The option is
 }
 ```
 
+### Main Field
+
+```javascript
+{
+  "mainFields": ["module", "main"]
+}
+```
+
+Quoting esbuild's documentation:
+
+* `main` - This is [the standard field](https://docs.npmjs.com/files/package.json#main) for all packages that are meant to be used with node. The name main is hard-coded in to node's module resolution logic itself. Because it's intended for use with node, it's reasonable to expect that the file path in this field is a CommonJS-style module.
+* `module` - This field came from a [proposal](https://github.com/dherman/defense-of-dot-js/blob/f31319be735b21739756b87d551f6711bd7aa283/proposal.md) for how to integrate ECMAScript modules into node. Because of this, it's reasonable to expect that the file path in this field is an ECMAScript-style module. This proposal wasn't adopted by node (node uses "type": "module" instead) but it was adopted by major bundlers because ECMAScript-style modules lead to better tree shaking, or dead code removal.
+* `browser` - This field came from a [proposal](https://gist.github.com/defunctzombie/4339901/49493836fb873ddaa4b8a7aa0ef2352119f69211) that allows bundlers to replace node-specific files or modules with their browser-friendly versions. It lets you specify an alternate browser-specific entry point. Note that it is possible for a package to use both the browser and module field together (see the note below).
+
 ### Errors
 
 * `Error: Package subpath '.' is not defined by "exports" in` - occurs when resolving without `conditionNames`.