Create topics/riscos.

A new topic for the expanding list of RISC OS components that have
been created.

Author: gerph

topics/riscos/index.md

@@ -0,0 +1,15 @@
+---
+display_name: RISC OS
+aliases: risc-os
+related: arm, bbc-basic
+logo: riscos.png
+released: September 1987
+short_description: RISC OS is the first ARM operating system.
+topic: riscos
+url: https://riscosopen.org/
+wikipedia_url: https://en.wikipedia.org/wiki/RISC_OS
+created_by: Acorn Computers Ltd
+---
+RISC OS is an operating system created by Acorn computers for the first ARM processors.
+It is a modular system, using a cooperative multitasking model. It is used by hobbyists
+on hardware such as the Raspberry Pi and Pinebook.