Update changelog

Author: timfel

CHANGELOG.md

@@ -3,6 +3,24 @@
 This changelog summarizes major changes between GraalVM versions of the Python
 language runtime. The main focus is on user-observable behavior of the engine.
 
+## Version 1.0.0 RC6
+
+* Support regular expression patterns built from bytes by using CPython's sre module as a fallback engine to our own
+* Support LLVM 5+ for C extension modules
+* Introduce native sequence storage so that e.g. Python bytes exposed to C can be mutated
+* Introduce lazy string concatenation to significantly speed up benchmarks where strings are concatenated repeatedly
+* C-API improvements to support more scikit-learn code
+* Fix our distinction between builtin functions, functions, and methods to make the classes for builtin functions equivalent to CPython
+* Improve set, frozenset, and dict support
+* Attach Python exceptions as cause to ImportErrors raised for C extension modules
+* Update standard library to CPython 3.6.5
+* Support more code object attributes
+* Support constant type ids for objects that are interned on CPython
+* Add collections.deque
+* Document how to contribute
+* Improve efficiency of generators
+* Enable re-use of ASTs in multiple Contexts in the same Engine
+
 ## Version 1.0.0 RC5
 
 * Generator expressions now properly evaluate their first iterator in the definition scope at definition time