Add OpenTitan to the list of users. [skip ci]

docs/markdown/Users.md

@@ -95,6 +95,7 @@ format files
  - [oomd](https://github.com/facebookincubator/oomd), a userspace Out-Of-Memory (OOM) killer for Linux systems
  - [OpenH264](https://github.com/cisco/openh264), open source H.264 codec
  - [OpenHMD](https://github.com/OpenHMD/OpenHMD), a free and open source API and drivers for immersive technology, such as head mounted displays with built in head tracking
+ - [OpenTitan](https://github.com/lowRISC/opentitan), an open source silicon Root of Trust (RoT) project.
  - [Orc](http://cgit.freedesktop.org/gstreamer/orc/), the Optimized Inner Loop Runtime Compiler
  - [OTS](https://github.com/khaledhosny/ots), the OpenType Sanitizer, parses and serializes OpenType files (OTF, TTF) and WOFF and WOFF2 font files, validating and sanitizing them as it goes. Used by Chromium and Firefox
  - [Outlier](https://github.com/kerolasa/outlier), a small Hello World style meson example project