Add RETURN sentence

Author: boriel

docs/return.md

@@ -0,0 +1,47 @@
+#RESTORE
+
+
+##Syntax
+```
+RETURN [<expr>]
+```
+**RETURN** statement is used in 3 cases:
+
+1. When returning from a [GO SUB](gosub.md) sentence
+2. When returning (exiting) from a [SUB](sub.md) (a subroutine)
+3. When returning (exiting) from a [FUNCTION](function.md). In this case a return value must be specified.
+
+
+Returns in the global scope (that is, outside any function or sub) are treated as return from [GO SUB](gosub.md).
+Otherwise they are considered as returning from the function or sub they are into.
+
+> **WARNING**: Using RETURN in global scope without a GOSUB will mostly crash your program. <br>
+> Use `--stack-check` if you suspect you have this bug, to detect it.
+
+### 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.
+
+##See also
+* [GO SUB](gosub.md)
+* [FUNCTION](function.md)
+* [SUB](sub.md)