It works

lib/readme.txt

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+
+This directory is intended for project specific (private) libraries.
+PlatformIO will compile them to static libraries and link them to executable files.
+
+The source code of each library should be placed in separate directories, like
+"lib/private_lib/[here are source files]".
+
+For example, see the structure of the following two libraries `Foo` and `Bar`:
+
+|--lib
+|  |
+|  |--Bar
+|  |  |--docs
+|  |  |--examples
+|  |  |--src
+|  |     |- Bar.c
+|  |     |- Bar.h
+|  |  |- library.json (optional, custom build options, etc) https://docs.platformio.org/page/librarymanager/config.html
+|  |
+|  |--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.
+
+More information about PlatformIO Library Dependency Finder
+- https://docs.platformio.org/page/librarymanager/ldf.html