Update doc to build sonar-python (#955)

Author: andrea-guarino-sonarsource

README.md

@@ -10,10 +10,20 @@
 
 ## Building the project
 
-sonar-python embeds [Typeshed](https://github.com/python/typeshed) as a Git submodule. Prior to building the project, you should therefore run `git submodule update --init` to retrieve the corresponding sources.
+Maven build is generating protobuf messages for Typeshed symbols from a python script (see [typeshed_serializer](https://github.com/SonarSource/sonar-python/tree/master/python-frontend/typeshed_serializer)).
+In order for it to work properly it needs to have Python runtime and [Typeshed](https://github.com/python/typeshed) available.
+
+### Prerequisites
+- Run `git submodule update --init` to retrieve [Typeshed](https://github.com/python/typeshed) as a Git submodule
+- Make sure to have Python 3.9 and [tox](https://tox.readthedocs.io/en/latest/) installed and available in PATH
+
+### Profiles
+
+- `mvn clean install` : execute full build, run tests for [typeshed_serializer](https://github.com/SonarSource/sonar-python/tree/master/python-frontend/typeshed_serializer)
+- `mvn clean install -DskipTypeshed`: avoid running [typeshed_serializer](https://github.com/SonarSource/sonar-python/tree/master/python-frontend/typeshed_serializer) tests and build only Java maven modules
 
 ## License
 
-Copyright 2011-2018 SonarSource.
+Copyright 2011-2021 SonarSource.
 
 Licensed under the [GNU Lesser General Public License, Version 3.0](http://www.gnu.org/licenses/lgpl.txt)