[doc] Mention python3 as a requirement for CAISAR to work properly.

doc/installation.rst

@@ -8,13 +8,15 @@ The latest release of CAISAR is available as an
 a `Docker <https://www.docker.com/>`_ image.
 
 The development version of CAISAR is available only by compiling the source code.
+CAISAR needs access to a ``python3`` interpreter, which is
+a default in all major GNU/Linux distributions.
 
 Install through Opam
 --------------------
 
-**Please note:** CAISAR requires the OCaml package manager
-(Opam) v2.1 or higher, which is typically avaible in all major
-GNU/Linux distributions.
+This method requires requires the OCaml package manager
+(Opam) v2.1 or higher to be installed in your system.
+It is typically avaible in all major GNU/Linux distributions.
 
 To install CAISAR via opam, do the following:
 
@@ -52,7 +54,9 @@ To run the CAISAR Docker image, do the following:
 Compile from source
 -------------------
 
-To build and install CAISAR, do the following:
+To build and install CAISAR, do the following (this may take
+some time, as it will download a full OCaml toolchain,
+required dependencies and compile them):
 
 .. code-block:: console