[docs] Improve YAML/README.md documentation

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

README.md

@@ -1,2 +1,54 @@
+# Install CIRCT
+
 Composite GitHub action to install [`llvm/circt`](https://github.com/llvm/circt)
 libraries and binaries.
+
+# Usage
+
+```yaml
+- name: Install CIRCT
+  uses: seldridge/install-circt@v1.0.0
+  with:
+    # The version of CIRCT to install.  This must match a release of CIRCT,
+    # e.g., 'firtool-1.56.1' or one of the following special strings:
+    #   - 'nightly' installs the latest nightly
+    #   - 'version-file' installs the release specified by a JSON file whose
+    #     location is set by the 'version-file' input.  This is done to
+    #     facilitate Continuous Delivery (CD) flows because GitHub Actions are
+    #     not allowed to modify anything in the '.github/' directory.
+    #
+    # Default: 'version-file'
+    version: ''
+
+    # A path to a JSON file that sets the CIRCT version to use.  This is
+    # required, but can be a dummy value if the user will not use the
+    # 'version-file' option for the 'version' input.
+    #
+    # The format of this JSON file is as follows where '<version>' should be
+    # replaced with a CIRCT release:
+    #
+    #     {
+    #       "version": "<version>"
+    #     }
+    #
+    # Default: './etc/circt.json'
+    version-file: ''
+
+    # The GitHub token used to download CIRCT nightly.
+    #
+    # Default: ${{ github.token }}
+    github-token: ''
+
+    # The name of the CIRCT tarball to install.  For available tarballs, see
+    # what is uploaded by a nightly build or by a release.  Example values are:
+    #   - 'circt-full-shared-linux-x64.tar.gz'
+    #   - 'firrtl-bin-macos-x64.tar.gz'
+    #
+    # Default: 'circt-full-shared-linux-x64.tar.gz'
+    file-name: ''
+
+    # The directory where CIRCT will be installed.
+    #
+    # Default: 'circt'
+    install-dir: ''
+```

action.yml

@@ -1,33 +1,43 @@
-name: Install CIRCT
+name: 'Install CIRCT'
+description: 'Install pre-built binaries and libraries from llvm/circt'
 
 inputs:
   version:
     description: |
-      The version of CIRCT to install.  This must match a release of CIRCT with
-      two exceptions used to indicate special versions:
-        - "nightly" installs the latest nightly
-        - "version-file" installs using a version specified by a JSON file whose
-          location is set by the "version-file" input.  This is done to
-          facilitate Continuous Delivery (CD) flows.
+      The version of CIRCT to install.  This must match a release of CIRCT,
+      e.g., 'firtool-1.56.1' or one of the following special strings:
+        - 'nightly' installs the latest nightly
+        - 'version-file' installs the release specified by a JSON file whose
+          location is set by the 'version-file' input.  This is done to
+          facilitate Continuous Delivery (CD) flows because GitHub Actions are
+          not allowed to modify anything in the '.github/' directory.
     required: true
     default: 'version-file'
   version-file:
     description: |
       A path to a JSON file that sets the CIRCT version to use.  This is
       required, but can be a dummy value if the user will not use the
-      "version-file" option for the "version" input.
+      'version-file' option for the 'version' input.
+
+      The format of this JSON file is as follows where '<version>' should be
+      replaced with a CIRCT release:
+
+          {
+            "version": "<version>"
+          }
     required: true
     default: './etc/circt.json'
   github-token:
     description: |
-      The GitHub token used to download CIRCT nightly.  It is unlikely that the
-      user needs to set this differently from the default.
+      The GitHub token used to download CIRCT nightly.
     required: true
     default: ${{ github.token }}
   file-name:
     description: |
       The name of the CIRCT tarball to install.  For available tarballs, see
-      what is uploaded by a nightly build or by a release.
+      what is uploaded by a nightly build or by a release.  Example values are:
+        - 'circt-full-shared-linux-x64.tar.gz'
+        - 'firrtl-bin-macos-x64.tar.gz'
     required: true
     default: 'circt-full-shared-linux-x64.tar.gz'
   install-dir: