Explain how to use "lib" directory from the PlatformIO based project // Issue #273

examples/atmelavr-and-arduino/arduino-external-libs/lib/readme.txt

@@ -0,0 +1,38 @@
+
+This directory is intended for the project specific (private) libraries.
+PlatformIO will compile them to static libraries and link to executable file.
+
+The source code of each library should be placed in separate directory, like
+"lib/private_lib/[here are source files]".
+
+For example, see how can be organised `Foo` and `Bar` libraries:
+
+|--lib
+|  |--Bar
+|  |  |--docs
+|  |  |--examples
+|  |  |--src
+|  |     |- Bar.c
+|  |     |- Bar.h
+|  |--Foo
+|  |  |- Foo.c
+|  |  |- Foo.h
+|  |- readme.txt --> THIS FILE
+|- platformio.ini
+|--src
+   |- main.c
+
+Then in `src/main.c` you should use:
+
+#include <Foo.h>
+#include <Bar.h>
+
+// rest H/C/CPP code
+
+PlatformIO will find your libraries automatically, configure preprocessor's
+include paths and build them.
+
+See additional options for PlatformIO Library Dependency Finder `lib_*`:
+
+http://docs.platformio.org/en/latest/projectconf.html#lib-install
+