DOC: update README (typo + update title)

README.md

-# Vagrant Démarches Simplifiées
+# Vagrant DEV for "Démarches Simplifiées"
 
-Vagrant box for Démarches Simplifiées (DS)
+Vagrant box for "Démarches Simplifiées" (DS)  software development.
 
 ## Prerequisites
 
-* Virtualbox 
-* Vagrant
-* Git
-* a Ruby version management tool like RVM or [RBenv](https://github.com/rbenv/rbenv)
-* r10k (`gem install r10k`)
+- [Vagrant](https://www.vagrantup.com/downloads)
+- [VirtualBox](https://www.virtualbox.org/)
+- Git
+- a Ruby version management tool like RVM or [RBenv](https://github.com/rbenv/rbenv)
+- r10k (`gem install r10k`)  
+
+For Linux users (debian, ubuntu), do *not* install Vagrant and VirtualBox from `apt-get` (packages are way too old),
+download `.deb` from the respective websites.
+
 
 ## Documentation
 
@@ -24,7 +28,7 @@ from an IDE (pending the use of Puppet to do this):
 - database user: `vagrant`
 - database password: `database1Password`
 
-```
+```shell
 vagrant ssh -c 'sudo su'
 
 ETC_PG_DIRECTORY="/etc/postgresql/10/main"