Update usage examples (#36)

* Use newer actions/checkout@v2 in examples

That's the current version. Even our own build scripts use it.

* Correct actions syntax

By the way, it's been ages since steps get their own section...

* Provide an example with matrix

Finally, have some example to showcase input usage.

* Move inputs after examples

Since they are more useful, put copy-pastable examples first. Then you
can look at all available inputs, now that you know how to use them.

README.md

@@ -8,14 +8,57 @@ This sets up the environment for compiling C/C++ code from command line.
 
 Supports Windows. Does nothing on Linux and macOS.
 
+## Example usage
+
+Basic usage for default compilation settings is like this:
+
+```yaml
+jobs:
+  test:
+    steps:
+      - uses: actions/checkout@v2
+      - uses: ilammy/msvc-dev-cmd@v1
+      - name: Build something requiring CL.EXE
+        run: |
+          cmake -G "NMake Makefiles" .
+          nmake
+      # ...
+```
+
+If you want something non-default,
+like using a specific version of Visual Studio,
+or cross-compling for a differen target,
+you will need to configure those settings via inputs:
+
+```yaml
+jobs:
+  test:
+    # Run a job for each of the specified target architectures:
+    strategy:
+      matrix:
+        arch:
+          - amd64
+          - amd64_x86
+          - amd64_arm64
+    steps:
+      - uses: actions/checkout@v2
+      - uses: ilammy/msvc-dev-cmd@v1
+        with:
+          arch: ${{ matrix.arch }}
+      - name: Build something requiring CL.EXE
+        run: |
+          cmake -G "NMake Makefiles" .
+          nmake
+      # ...
+```
+
 ## Inputs
 
 - `arch` – target architecture
   - native compilation:
     - `x64` (default) or its synonyms: `amd64`, `win64`
     - `x86` or its synonyms: `win32`
-  - cross-compilation: `x86_amd64`, `x86_arm`, `x86_arm64`,
-  	`amd64_x86`, `amd64_arm`, `amd64_arm64`
+  - 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`)
@@ -28,20 +71,6 @@ Supports Windows. Does nothing on Linux and macOS.
 - `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
-    # ...
-```
-
 ## Caveats
 
 ### Name conflicts with `shell: bash`