Add CHANGELOG entries for 1.0.0 RC3.

Author: nirvdrum

CHANGELOG.md

@@ -23,6 +23,10 @@ Performance:
   avoiding extra allocations.
 * `rb_str_new()`/`rb_str_new_cstr()` are much faster by avoiding extra copying and
   allocations.
+* `String#{sub,sub!}` are faster in the common case of an empty replacement string.
+* Eliminated many unnecessary memory copy operations when reading from `IO` with
+  a delimiter (e.g., `IO#each`), leading to overall improved `IO` reading for common
+  use cases such as iterating through lines in a `File`.
 
 # 1.0 RC 2, May 2018