Mention B-ext support in README

Author: jerryz123

README.md

@@ -1,7 +1,7 @@
 Shuttle: A Rocket-based Superscalar In-order RISC-V Core
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-Shuttle is a Rocket-based superscalar in-order RISC-V core, supporting the base RV64IMAFDC instruction set with supervisor and user-mode.
+Shuttle is a Rocket-based superscalar in-order RISC-V core, supporting the base RV64IMAFDCB instruction set with supervisor and user-mode.
 Shuttle is a 6-stage core that can be configured to be dual, three, or quad-issue, although dual-issue is the most sensible design point.
 Shuttle is *not* designed to meet any power, performance, or area targets.
 It exists purely as a demonstrative example of another RISC-V CPU design point.