docs, mention that makefiles can be used as project

docs/features_projects.md

@@ -3,7 +3,7 @@ title: Projects
 header-includes: <script src="https://cdnjs.cloudflare.com/ajax/libs/anchor-js/4.2.2/anchor.min.js"></script>
 ---
 
-Two project formats are supported. 
+Three project formats are supported.
 For all the formats a single widget is used to display the source files list and the configurations, the [project inspector](widgets_project_inspector.html).
 Project properties are edited in specific widgets.
 
@@ -18,6 +18,29 @@ The widget used to edit the properties is the [DUB project editor](widgets_dub_p
 
 The option editor exposes a [category for DUB](options_dub_build.html). The options specified in this category are applied each time a project is compiled.
 
+## Makefiles
+
+Makefiles can be opened as projects.
+
+The rules found in the makefile are interpreted as configurations.
+
+Source files are detected using a list of prefixes.
+Files located in a sub folder and that start with one of the prefix is then displayed in the project inspector.
+Custom prefixes can be specified in the _Options_ menu, category _Makefile projects_.
+Default prefixes are "src", "import", and "include".
+
+While not very useful for D, Makefile projects allow to use dexed with other languages.
+Working IDE features are then limited to
+
+- Compile project
+- The [messages](widgets_messages.html) reported by _make_ can be clicked to open new files
+- [Custom tools](widgets_custom_tools.html) can use symbolic strings, allowing for example to setup a custom linter for all the sources detected using the prefixes
+- [Searches](widgets_search.html) in the whole project
+- Text completions (based on work-splitting)
+- Integration in [project groups](widgets_project_groups.html)
+
+Note that the dialog used to open a project will only recognize "Makefile" and files with the ".mak" extension.
+
 ## DEXED format
 
 The DEXED format (previously called _native format_ then _ce projects_) is based on DMD command line interface.