"Library Directories Structure" chapter added

docs/CMakeLists.txt

@@ -65,6 +65,7 @@ add_custom_command(OUTPUT "${SPHINX_INDEX_FILE}"
                        "${CMAKE_CURRENT_SOURCE_DIR}/_static/css/custom.css"
                        "${CMAKE_CURRENT_SOURCE_DIR}/CHANGELOG.md"
                        "${CMAKE_CURRENT_SOURCE_DIR}/design.rst"
+                       "${CMAKE_CURRENT_SOURCE_DIR}/design/directories.rst"
                        "${CMAKE_CURRENT_SOURCE_DIR}/design/quantity.rst"
                        "${CMAKE_CURRENT_SOURCE_DIR}/examples.rst"
                        "${CMAKE_CURRENT_SOURCE_DIR}/examples/hello_units.rst"

docs/design.rst

@@ -10,6 +10,7 @@ Design
 .. toctree::
     :maxdepth: 2
 
+    design/directories
     design/quantity
 
 The Downcasting Facility

docs/design/directories.rst

@@ -0,0 +1,48 @@
+.. namespace:: units
+
+Library Directories Structure
+=============================
+
+.. code-block:: text
+
+    units
+    ├── bits
+    │   └── external
+    ├── data
+    └── physical
+        ├── cgs
+        ├── iau
+        ├── imperial
+        ├── international
+        ├── natural
+        ├── si
+        ├── typographic
+        └── us
+
+- *./units*
+
+  - The main directory of the library.
+  - Contains headers files that define a public interface of the library framework
+
+- *./units/bits*
+
+  - Contains header files with implementation details for the library. Interface of
+    the tools provided here should not be standardized.
+
+- *./units/bits/external*
+
+  - Contains header files of general purpose utilities that are not necessary
+    **mp-units** library specific. They are either implementation details of the
+    library or should be (or already are) the subject of separate standardization
+    proposals not related to a Physical Units library proposal.
+
+- *./units/physical*
+
+  - Contains the definition of physical units dimensions
+  - Its subfolders provide the definitions of various
+    :term:`systems of units <system of units>` with :term:`SI` being the most popular
+    one.
+
+- *./units/data*
+
+  - Provides data information dimensions (i.e. `data::bitrate`).