Initial action stub

Now that I know how to write some simple action, let's make a stub
more complete. For example, I already know what inputs I would like
to handle for this action.

README.md

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+<a href="https://github.com/ilammy/msvc-dev-cmd"><img alt="GitHub Actions status" src="https://github.com/ilammy/msvc-dev-cmd/workflows/setup-nasm/badge.svg"></a>
+
+# msvc-dev-cmd
+
+[GitHub Action](https://github.com/features/actions) for configuring Developer Command Prompt for Microsoft Visual C++.
+
+This sets up the environment for compiling C/C++ code from command line.
+
+Supports Windows. Does nothing on Linux and macOS.
+
+## Inputs
+
+- `version` – version of NASM to install (default: 2.14.02)
+- `from-source` – set to `true` to always build from source, or `false` to never
+- `platform` – set binary platform to something non-standard
+- `destination` – target directory for download and installation (default: `$HOME/nasm`)
+
+Set true to build for Universal Windows Platform (i.e., for Windows Store)
+
+- `arch` – target architecture
+  - native compilation: `x86`, `x64` (default), `amd64` (synonym for x64)
+  - cross-compilation: `x86_amd64`, `x86_arm`, `x86_arm64`,
+  	`amd64_x86`, `amd64_arm`, `amd64_arm64`
+- `sdk` – Windows SDK to use
+  - do not specify to use the default SDK
+  - or specify full Windows 10 SDK number (e.g, `10.0.10240.0`)
+  - or write `8.1` to use Windows 8.1 SDK
+- `toolset` – select VC++ compiler toolset version
+  - do not specify to use the default toolset
+  - `14.0` for VC++ 2015 Compiler Toolset
+  - `14.XX` for the latest 14.XX toolset installed (e.g, `14.11`)
+  - `14.XX.YYYYY` for a specific full version number (e.g, `14.11.25503`)
+- `uwp` – set `true` to build for Universal Windows Platform (i.e., for Windows Store)
+- `spectre` – set `true` to use Visual Studio libraries with [Spectre](https://meltdownattack.com) mitigations
+
+## Example usage
+
+```yaml
+jobs:
+  test:
+    - uses: actions/checkout@v1
+    - uses: ilammy/msvc-dev-cmd@v1
+    - name: Build something requiring CL.EXE
+      run: |
+        cmake -G "NMake Makefiles" .
+        nmake
+    # ...
+```
+
+## License
+
+MIT, see [LICENSE](LICENSE).