documented how to create hybrid ROM

bin/README.md

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+
+Follow these rules, or bad things will happen!
+
+
+* Don't mix ROMs for different platforms - use generic BASIC with generic KERNAL, Ultimate64 BASIC with Ultimate64 KERNAL, etc.
+
+* Don't mix these ROMs with any other ROMs; exceptions:
+
+   - kernal_hybrid.rom, which is meant to be used (and has to be used) with the original BASIC ROM;
+     however, this hybrid ROM cannot be distributed due to legal/copyright reasons; see [here](./../doc/Hybrid-build.md) for instruction how to create it
+
+   - chargen_*.rom - these ones are fully interchangeable
+
+- Don't mix ROMs from different builds/versions (again, CHARGEN is an exception).
+
+- The generic ROM set should be compatible with any C64-compatible machine; platform-specific ROMs might not work on other platforms.
+
+
+Have fun!
+
+
+------
+
+
+PXL font was created by Retrofan, we got a permission to include it with Open ROMs under GNU Lesser General Public License 3.0, see [here](https://www.forum64.de/index.php?thread/84000-neuer-c64-systemfont-auch-plus4-c128-atari/&postID=1493383#post1493383).