add Chapter 15 - runtime observer implementation

Chapter15/observer/libs/etl/examples/platformio/platformio.ini

@@ -0,0 +1,27 @@
+; PlatformIO Project Configuration File
+;
+;   Build options: build flags, source filter
+;   Upload options: custom upload port, speed and extra flags
+;   Library options: dependencies, extra library storages
+;   Advanced options: extra scripting
+;
+; Please visit documentation for the other options and examples
+; http://docs.platformio.org/page/projectconf.html
+
+[platformio]
+default_envs = native
+
+[env:native]
+platform = native
+build_flags =
+  ; ETL configs in `include` folder are minimalistic. Here we can set all
+  ; additional definitions to keep everything in one place and customize values
+  ; for different target platforms
+  -D PROFILE_GCC_GENERIC
+lib_deps =
+  ; Define ETL dependency for this demo. You can use versions from PIO registry,
+  ; or git repository with specific branch, tag or commit. See PIO docs for
+  ; details.
+  ;Embedded Template Library=https://github.com/ETLCPP/etl/archive/master.zip
+  Embedded Template Library@^14.31.2
+