AP_OSD: Addded README.md for fonts

libraries/AP_OSD/fonts/README.md

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+# Font styles
+
+_The implementation of on-the-fly font style switching is still work in progress. As for now, there's a variety of font styles that can be chosen prior to building the code._
+
+Credits to [inavosd](https://github.com/iNavFlight/inav-configurator/tree/master/resources/osd) for providing the basis to develop on.
+
+Available font styles are:
+
+**clarity (default)** a clearly visible large font:
+
+  ![](clarity.png)
+
+**clarity_medium** good visibility, less dominant than clarity default:
+
+![](clarity_medium.png)
+
+**bfstyle** bf- / inav-osd default style: 
+
+![](bfstyle.png)
+
+**bold** a bolder version of bfstyle:
+
+![](bold.png)
+
+**digital** 80's clock radio style:
+
+![](digital.png)
+
+
+
+## Individualizing charsets ##
+
+Charsets can be edited or built form scratch using [MAX7456charwizard](https://github.com/diydrones/MinimOSD-Extra/blob/master/Tools/MAX7456Charwizard.jar)
+
+
+
+## Building a firmware with a non-default font style
+
+_On-the-fly style switching being still-work-in progress, as for now it is required to convert the desired .mcm file to a file named clarity.bin that is automatically integrated into the build._
+
+In your cloned repository navigate to this directory:
+```
+/ardupilot/libraries/AP_OSD/fonts
+```
+and call "mcm2bin.py input.mcm output.bin" to convert your chosen .mcm file:
+```
+./mcm2bin.py yourfont.mcm clarity.bin
+```
+Now navigate back to the directory root, configure and build for your desired board.