Updating changelog to include .NET Core support

CHANGELOG.txt

@@ -1,4 +1,5 @@
-### 1.10.1 (unreleased)
+### 2.0.0 (unreleased)
+* [NEW] Initial .NET Core support. Thanks to @alexandrnikitin for tonnes of work on this. Thanks also to Peter Jas (@jasonwilliams200OK) for both .NET Core code contributions and help with CI setup. And thanks to everyone who contributed to the conversation and testing in https://github.com/nsubstitute/NSubstitute/pull/197.
 * [NEW] Clear substitute configuration using `sub.ClearSubstitute(ClearOptions)`. This extension can be brought in to scope via `using NSubstitute.ClearExtensions;`. Thanks to @trullock for defining and implementing this change, and to @alexandrnikitin for review feedback. (#157, #235, #179)
 * [DOC] Improved CallInfo.ArgAt documentation, thanks to asbjornu. (#231)