[Docs] Expand a bit on project detection

doc/modules/ROOT/pages/projects.adoc

@@ -6,6 +6,19 @@ One of the main goals of Projectile is to operate on a wide range of project typ
 without the need for any configuration. To achieve this it contains a lot of
 project detection logic and project type specific logic.
 
+Broadly speaking, Projectile identifies projects like this:
+
+* Directories under version control (e.g. a Git repo)
+* Directories that contain some project description file (e.g. a `Gemfile` for Ruby projects)
+* Directories that contain the special `.projectile` file
+
+While Projectile aims to recognize most project types out-of-the-box, it's also extremely
+flexible configuration-wise, and you can easily alter the project detection logic.
+
+TIP: If you'd like to override the default project detection functions you should
+check out `projectile-project-root-files-functions`. We'll discuss how to tweak in more
+details later in the documentation.
+
 === Version Control Systems
 
 Projectile considers most version-controlled repos to be