docs: add section about compiler directives

doc/pascalprogramming.md

@@ -135,3 +135,67 @@ var c:char;
     ...
 
 ```
+
+## Compiler Directives
+
+Tridora-Pascal understands a small number of compiler directives which are introduced as usual with a comment and a dollar-sign. Both comment styles can be used.
+All directives use a single, case-sensitive letter.
+
+### Include (_I_)
+The _Include_ directive reads another source file at compile time. Includes can be nested (that is, an included file can include other files). After the directive letter _I_,
+a string must be specified that indicates the file name.
+
+Example:
+```
+program includetest;
+
+{$I 'morestuff.pas'}
+
+...
+
+```
+
+The compiler looks for the file first on the default volume, then on the _SYSTEM_ volume. The file name must be enclosed in single quotes.
+
+If the file name contains a _+_ character, the string 'tdr' is inserted after that _+_ character. This is used for compiling the cross-compiler.
+FPC will use the original filename, Tridora Pascal will use the name with 'tdr' inserted.
+
+### Set Heap Size (_H_)
+Sets the heap size for the compiled program. After the directive letter _H_, an integer must be specified that represents the heap size
+in kilobytes. The default value is 256.
+
+The directive must appear before the _program_ keyword to have an effect.
+
+Example:
+```
+{$H 800}
+program bigHeap;
+
+...
+
+```
+### Set Stack Size (_S_)
+Sets the stack size for the compiled program. After the directive letter _S_, an integer must be specified that represents the stack size
+in kilobytes. The default value is 16.
+
+The directive must appear before the _program_ keyword to have an effect.
+
+Example:
+```
+{$S 128}
+program deepRecursion;
+
+...
+
+```
+
+### Ignore Rest of Line (_!_)
+This directive is a hack to make the Tridora compiler ignore directives meant for other compilers. Used for compiling the cross-compiler.
+
+This directive uses the character _!_. After this directive, the rest of the line is not parsed. The closing comment is still required.
+
+Example:
+```
+{$!}uses math,crt;
+```
+In this example, the line is ignored by the Tridora compiler but FPC will parse it.