Add ChangeLog entry for library/makefile changes

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index c0759b2..4985bcc 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -10,6 +10,10 @@
    * Remove some redundant code in bignum.c. Contributed by Alexey Skalozub.
    * Support cmake build where Mbed TLS is a subproject. Fix
      contributed independently by Matthieu Volat and Arne Schwabe.
+   * Allow configuring the prefix operator for the archiver tool when compiling
+     the library using the makefile. Found and fixed by Alex Hixon.
+   * Allow configuring the shared library extension by setting the DLEXT
+     variable when using the project makefile.
 
 = mbed TLS 2.8.0 branch released 2018-03-16