docs(ostag): add ostag input to README (#25)

#24 neglected to add the new ostag input to the README

Author: wpbonelli

README.md

@@ -9,7 +9,6 @@ An action to setup MODFLOW 6 and related programs.
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-
 - [Overview](#overview)
 - [Usage](#usage)
 - [Environment variables](#environment-variables)
@@ -21,6 +20,7 @@ An action to setup MODFLOW 6 and related programs.
   - [`tag`](#tag)
   - [`subset`](#subset)
   - [`cache`](#cache)
+  - [`ostag`](#ostag)
 - [Outputs](#outputs)
   - [`cache-hit`](#cache-hit)
 - [MODFLOW Resources](#modflow-resources)
@@ -112,6 +112,10 @@ The `cache` input is a boolean that controls whether the action caches the MODFL
 
 **Note:** an [outstanding cache reservation bug in `actions/cache`](https://github.com/actions/cache/issues/144) can cause the cache to [fail to restore while simultaneously rejecting new saves](https://github.com/MODFLOW-USGS/modflow6/actions/runs/3624583228/jobs/6111766806#step:6:152). The [GitHub-endorsed workaround for this issue](https://github.com/actions/cache/issues/144#issuecomment-579323937) is currently to change keys, therefore this action rotates the cache key once daily. Rotating the key daily also allows the action to automatically update versions when they become available, e.g. so workflows can use the most recent nightly build.
 
+### `ostag`
+
+The `ostag` input allows selecting a release by operating system. By default, this is the system the action is running on. Typically one will not need to override this default &mdash; one reason to do so is to select an Intel macOS distribution on an ARM macOS runner.
+
 ## Outputs
 
 The action has the following outputs: