docs: add structure doc

Signed-off-by: perigoso <perigoso@riseup.net>

docs/contributing/structure.rst

@@ -0,0 +1,59 @@
+*********
+Structure
+*********
+
+Tree
+====
+
+::
+
+  .
+  ├── docs
+  ├── examples
+  ├── hw
+  │   ├── bsp
+  │   └── mcu
+  ├── lib
+  ├── src
+  ├── test
+  └── tools
+
+docs
+----
+
+Documentation
+
+examples
+--------
+
+Sample with Makefile build support
+
+hw/bsp
+------
+
+Supported boards source files
+
+hw/mcu
+------
+
+Low level mcu core & peripheral drivers
+
+lib
+---
+
+Sources from 3rd party such as freeRTOS, fatfs ...
+
+src
+---
+
+All sources files for TinyUSB stack itself.
+
+test
+----
+
+Unit tests for the stack
+
+tools
+-----
+
+Files used internally