Add "Hello World" example for desktop platforms

examples/desktop/hello-world/README.rst

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+How to build PlatformIO based project
+=====================================
+
+1. `Install PlatformIO <http://docs.platformio.org/en/latest/installation.html>`_
+2. Download `source code with examples <https://github.com/platformio/platformio/archive/develop.zip>`_
+3. Extract ZIP archive
+4. Run these commands:
+
+.. code-block:: bash
+
+    # Change directory to example
+    > cd platformio-develop/examples/desktop/hello-world
+
+    # Process example project
+    > platformio run
+
+    # Clean build files
+    > platformio run --target clean

examples/desktop/hello-world/platformio.ini

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+#
+# Project Configuration File
+#
+# A detailed documentation with the EXAMPLES is located here:
+# http://docs.platformio.org/en/latest/projectconf.html
+#
+
+# A sign `#` at the beginning of the line indicates a comment
+# Comment lines are ignored.
+
+# Simple and base environment
+# [env:mybaseenv]
+# platform = %INSTALLED_PLATFORM_NAME_HERE%
+# framework =
+# board =
+#
+# Automatic targets - enable auto-uploading
+# targets = upload
+
+[env:native]
+platform = native
+
+[env:win32]
+platform = windows_x86

examples/desktop/hello-world/src/main.c

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+#include <stdio.h>
+
+int main()
+{
+    printf("Hello World from PlatformIO!\n");
+    return 0;
+}