doc: add END doc

Author: boriel

docs/end.md

@@ -0,0 +1,19 @@
+# END
+
+
+##Syntax
+```
+END [<value>]
+```
+
+Terminates execution and returns to the Operating System (i.e. to the Sinclair
+BASIC interpreter). An optional value can be used (defaults to 0 if not specified)
+that will be returned to the OS.
+
+```basic
+PRINT "HELLO WORLD"
+END 32: REM The value 32 will be returned to the OS
+```
+
+End is also a keyword used to close [scopes](scope.md) in [FUNCTION](function.md) and [SUB](sub.md)
+and compound sentences in [IF](if.md), [WHILE](while.md).