docs: added some concept processing instructions

I might implement these later, or not I have not decided yet.

Author: Tygo-van-den-Hurk

docs/processing-instructions.md

@@ -20,7 +20,9 @@ An example is the processing instructions to change the [markup renderer](markup
 
 Here follows a list off all possible instructions:
 
-- `markup`: change the active markup renderer
+- [`markup`](#change-markup-renderer): change the active markup renderer
+- [`engine`](#engine-concept): checks if the compiler uses the same version for slyde.
+- [`include`](#include-concept): include files in the final build.
 
 ### Change Markup Renderer
 
@@ -29,3 +31,30 @@ You can change the markup render to `XYZ` using:
 ```XML
 <?slyde markup="XYZ"?>
 ```
+
+## Engine (concept)
+
+> [!NOTE]
+> For now this is just a concept, I am not sure if it will make it into the final version.
+
+Require a certain version for slyde to be. Works using semantic versioning.
+
+```
+<?slyde engine="v1"?>
+<?slyde engine="v1.2"?>
+<?slyde engine="v1.2.3"?>
+```
+
+## Include (concept)
+
+> [!NOTE]
+> For now this is just a concept, I am not sure if it will make it into the final version.
+
+You can include different files inside the final result. The parser is smart enough to know what type of content it is, and where to put it.
+
+```XML
+<?slyde include="http://example.com/script.js"?>
+<?slyde include="./path/to/script.js"?>
+<?slyde include="http://example.com/style.css"?>
+<?slyde include="./path/to/style.css"?>
+```