Add target audience and prior knowledge

Author: rcvalle

src/doc/rustc/src/exploit-mitigations.md

@@ -1,5 +1,14 @@
 # Exploit Mitigations
 
+This chapter describes exploit mitigations supported by the Rust compiler,
+and is by no means an extensive survey of the Rust programming language’s
+security features.
+
+This chapter is for software engineers working with the Rust programming
+language, and assumes prior knowledge of the Rust programming language and
+its toolchain.
+
+
 ## Introduction
 
 The Rust programming language provides memory[1] and thread[2] safety