Add README.MD file.

README.MD

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+ZSM
+===
+
+ZSM, also known as Z80ASMUK, is a Z80 assembler for CP/M, that outputs HEX files.
+
+I like so much this assembler, so I have recopiled every ZSM version I could find over the Internet and my own disks.
+
+I have even contributed a bit to ZSM with code of my own, mainly patches.
+
+![ZSM image](http://www.floppysoftware.es/images/cpm-zsm.jpg "ZSM image")
+
+Additional tools
+----------------
+
+I have developed some companion tools, in order to have a good native development tool chain for the Z80 assembler programming language under the CP/M operating system:
+- `zsmpp`: source code preprocessor for the ZSM assembler.
+- `bintoasm`: converts a binary file into an assembler file for ZSM.
+- `hextobin`: converts a HEX file into a binary file.
+- `hextocom`: converts an HEX file into a COM file for CP/M.
+- `prl`: makes PRL relocatable files from COM files.
+- `prntosym`: extracts symbols from a PRN file (from ZSM v2.8+) and generates an assembler file for ZSM.
+- `rsx`: manages RSX files.
+- `dump`: prints file contents in hexadecimal and ascii format.
+
+License
+-------
+
+Reading the source code of ZSM, it seems there are various custom licenses involved.
+
+Regarding my companion tools, they are under the GPL license.