Add eachindex to README

Author: timholy

Diff for: README.md

@@ -50,6 +50,10 @@ Currently, the `@compat` macro supports the following syntaxes:
 
 * For all unsigned integer types to their equivalents with uppercase `I`. [#8907](https://github.com/JuliaLang/julia/pull/8907)
 
+## New functions
+
+* `eachindex`, as in `for i in eachindex(A)`, can be used in julia 0.3. This is the recommended way to iterate over each index in an `AbstractArray`. On julia 0.3 `eachindex` just returns `1:length(A)`, but in julia 0.4 it can return a more sophisticated iterator. 
+
 ## Renamed functions
 
 * `itrunc`, `iround`, `iceil`, `ifloor` are now accessed via `trunc(T, x)`, etc. [#9133](https://github.com/JuliaLang/julia/pull/9133)