update doc to clarify build procedure & python dependencies

Author: marc-joos

README.md

@@ -39,7 +39,9 @@ Interface, for building an universal MPI code:
 
 * **GNU/Linux OS**
 * **Mandatory**: C and Fortran compilers
-* **Optionnal**: Python3 and Jinja2
+* **Optional**: Python 3 and Jinja 2
+
+Python 3 is mandatory if you need to generate Wi4MPI code. As specified in the [generator requirements](src/generator/requirements.txt), it depends on `logging`, `colorlog` & `docopts` Python packages.
 
 ### Spack
 
@@ -53,7 +55,8 @@ Check the WI4MPI Spack tutorial here: [Installalling Wi4MPI using Spack](https:/
 
 ### Compiling with CMAKE
 
-Here is a quick introduction to WI4MPI compiling, please check the full documentation to see other options.
+Here is a quick introduction to compile WI4MPI with pre-generated sources (using `wi4mpi-x.y.z.tar.gz` archive 
+provided with releases, starting with Wi4MPI 4.0):
 
 ```
 $ cd build
@@ -62,6 +65,8 @@ $ make -j 16
 $ make install
 ```
 
+For more details (and in particular if you need to regenerate the sources) please check the full documentation.
+
 ### Documentation
 
 Wi4MPI documentation is built with `Sphinx`, choose your preferred format.

doc/source/user_guide/installation.rst

@@ -2,7 +2,7 @@ Installation
 ============
 
 The installation process consists of a standard CMake build and installation sequence.
-The release archive contains pre-generated code to simplify the build process.
+The release archive contains pre-generated code (`wi4mpi-x.y.z.tar.gz` archive, containing the generated code in `build/`) to simplify the build process. By default, the build process expects this generated sources to be there (`WI4MPI_GENERATOR` is set to `OFF`).
 
 .. code-block:: bash
 
@@ -47,14 +47,14 @@ The following CMake variables can be used to configure the build:
 
   Specifies the Open MPI header version used during code generation.
 
-  **Default:** `1.8.8`
+  **Default:** `4.1.6`
   **Supported versions:** `1.8.8`, `2.1.6`, `4.1.6`, `5.0.3`
 
 - **WI4MPI_GENERATOR_MPICH_VERSION** (*string*)
 
   Specifies the MPICH header version used during code generation.
 
-  **Default:** `3.1.2`
+  **Default:** `3.4.3`
   **Supported versions:** `3.1.2`, `3.4.3`, `4.2.0`
 
 - **WI4MPI_GENERATOR_INTELMPI_VERSION** (*string*)