Add install for flex and bison to satisfy LEX and YACC requirements

Change-Id: I5f6a527d83af5eca91dcda6ec8e415c48c34da32
diff --git a/docs/GettingStarted.md b/docs/GettingStarted.md
index ff9023e..9ab08f4 100644
--- a/docs/GettingStarted.md
+++ b/docs/GettingStarted.md
@@ -22,7 +22,7 @@
 Install prerequisites:
 
 ``` shell
-sudo apt install make binutils-aarch64-linux-gnu device-tree-compiler libssl-dev
+sudo apt install make binutils-aarch64-linux-gnu device-tree-compiler libssl-dev flex bison
 ```
 
 By default, the hypervisor is built with clang for a few target platforms along