Merge pull request #32 from shepmaster/patch-2

Define "interior mutability"

Author: avadacatavra

reference/src/introduction.md

@@ -3,6 +3,8 @@
 
 ## Terms
 
+* *interior mutability* — mutating memory where there also exists a live shared reference non-transitively pointing to the same memory.
+
 (TODO: definitions)
 
 * *niche*