Add library.json

Adds the library.json metadata file [1] so that tinyfsm can be used in
the PlatformIO build system and IDE [2] by means of declaring this
repo as a library dependency [3].

PlatformIO is a versatile Python-based tool and VSCode extension that
allows building for over 800 microcontroller boards [4] of all
different architectures and frameworks like Arduino, mbed-os, Zephyr,
etc.

  [1]: https://docs.platformio.org/en/latest/librarymanager/config.html
  [2]: https://docs.platformio.org/en/latest/integration/ide/pioide.html
  [3]: https://docs.platformio.org/en/latest/projectconf/section_env_library.html#lib-deps
  [4]: https://platformio.org/boards

library.json

@@ -0,0 +1,24 @@
+{
+    "name": "tinyfsm",
+    "description": "A simple C++ finite state machine library",
+    "version": "0.3.2",
+    "repository": {
+        "type": "git",
+        "url": "https://dev.tty0.ch/tinyfsm.git"
+    },
+    "homepage": "https://digint.ch/tinyfsm/",
+    "authors": {
+        "name": "Axel Burri",
+        "email": "[email protected]",
+        "url": "http://digint.ch",
+        "maintainer": true
+    },
+    "license": "MIT",
+    "platforms": "*",
+    "frameworks": "*",
+    "build": {
+        "flags": [
+            "-I include/"
+        ]
+    }
+}