core: add helpers for working with paths across OSes (#1102)

packages/core/README.md

@@ -309,4 +309,27 @@ outputs:
 runs:
   using: 'node12'
   main: 'dist/index.js'
-```
+```
+
+#### Filesystem path helpers
+
+You can use these methods to manipulate file paths across operating systems.
+
+The `toPosixPath` function converts input paths to Posix-style (Linux) paths.
+The `toWin32Path` function converts input paths to Windows-style paths. These
+functions work independently of the underlying runner operating system.
+
+```js
+toPosixPath('\\foo\\bar') // => /foo/bar
+toWin32Path('/foo/bar') // => \foo\bar
+```
+
+The `toPlatformPath` function converts input paths to the expected value on the runner's operating system.
+
+```js
+// On a Windows runner.
+toPlatformPath('/foo/bar') // => \foo\bar
+
+// On a Linux runner.
+toPlatformPath('\\foo\\bar') // => /foo/bar
+```