update README

Author: msaarna

README.md

@@ -1 +1,5 @@
 # a78_asm_header
+
+This file can be used to add an a78 header to the rom produced by your assembly source code.
+
+If you wish to add/modify the header on an already assembled rom, check out the 7800header utility, which can be found in the 7800 Assembly Dev Kit (http://7800.8bitdev.org/index.php/7800AsmDevKit)