README: update

README.md

@@ -5,13 +5,19 @@ Set up Clang
 
 This GitHub action sets up Clang & LLVM in your workflow run.
 
+1. Installs either 32-bit or 64-bit Clang & LLVM on either Ubuntu, Windows or
+Cygwin.
+2. Specify a version to install using the `version` parameter.
+
 Use it in your workflow like this:
 
     - name: Set up Clang
       uses: egor-tensin/setup-clang@v1
       with:
+        version: latest
         platform: x64
 
+* `latest` is the default value for the `version` parameter and can be omitted.
 * `x64` is the default value for the `platform` parameter and can be omitted.
 Use `x86` if you want to build 32-bit binaries.
 * Set the `cygwin` parameter to `1` to set up Clang inside an existing Cygwin
@@ -27,6 +33,8 @@ API
 
 | Input     | Value   | Default | Description
 | --------- | ------- | ------- | -----------
+| version   | latest  | ✓       | Install the latest version available in the repository.
+|           | *any*   |         | Install a specific version if it's available (see below).
 | platform  | x64     | ✓       | Install the x86_64 toolchain.
 |           | *any*   |         | Install the i686 toolchain.
 | cygwin    | *any*   | ✓       | Install native binaries.
@@ -36,6 +44,39 @@ API
 | hardlinks | *any*   | ✓       | Cygwin: don't convert any symlinks.
 |           | 1       |         | Cygwin: convert symlinks in /usr/bin to hardlinks.
 
+Supported versions
+------------------
+
+Unless the `version` parameter value is "latest", the official LLVM repository
+is used to make more versions available.
+You can pass the version number as the `version` parameter value (`5.0`, `8`,
+`9`, etc.), and this action will install the corresponding packages.
+
+The `version` parameter value is not checked for being an available version for
+the current distribution.
+The supported versions for a particular distribution are those found in that
+distro's repositories & those in the LLVM repository.
+For example, the supported versions for Bionic & Focal as of July 2021 are
+listed below.
+
+| `version` | Bionic | Focal
+| --------- | ------ | -----
+| 3.9       | ✓      |
+| 4.0       | ✓      |
+| 5.0       | ✓      |
+| 6.0       | ✓      | ✓
+| 7         | ✓      | ✓
+| 8         | ✓      | ✓
+| 9         | ✓      | ✓
+| 10        | ✓      | ✓
+| 11        | ✓      | ✓
+| 12        | ✓      | ✓
+
+This table is not definitive; I expect more future versions to be made
+available, especially for Focal.
+
+On Windows and Cygwin, the `version` parameter is ignored.
+
 License
 -------