doc: add GO SUB page

Author: boriel

docs/gosub.md

@@ -0,0 +1,57 @@
+#GO SUB
+
+
+##Syntax
+```
+GO SUB <label>
+GOSUB <label>
+```
+Continues the execution at the given label or line number.
+The current execution point is pushed (stored) onto the stack,
+to be recovered later.
+
+When a [RETURN](return.md) is found and executed, the previous
+execution point is popped out (recovered) from the stack and
+continues just after the GO SUB. This is a way to create simple
+subroutines.
+
+This sentence exists just for compatibility with legacy BASIC
+dialects. You should use [SUB](sub.md) or [FUNCTION](function.md) instead.
+
+GO SUB cannot be used within neither subroutines nor functions.
+You can't GOSUB into a function or sub. So GOSUB is limited to
+global [scope](scope.md)
+
+> **WARNING**: Using GO SUB continuously without returning with
+> RETURN will eventually fill the stack (stack overflow) and crash
+> your program.
+
+### Example with GO SUB
+
+```
+10 LET number = 10
+20 GOSUB 1000 : REM calls the subroutine
+30 LET number = 20
+40 GOSUB 1000 : REM calls the subroutine again
+100  END : REM the program must end here to avoid entering the subroutine without using GOSUB
+1000 REM Subroutine that prints number + 1
+1010 PRINT "number + 1 is "; number + 1
+1020 RETURN : REM return to the caller
+```
+
+This will output:
+
+```
+number + 1 is 11
+number + 1 is 21
+```
+
+
+##Remarks
+* This statement is Sinclair BASIC compatible.
+* GO SUB cannot be used within subrutines nor functions.
+
+##See also
+* [RETURN](return.md)
+* [FUNCTION](function.md)
+* [SUB](sub.md)

docs/return.md

@@ -1,4 +1,4 @@
-#RESTORE
+#RETURN
 
 
 ##Syntax