Add a CHANGELOG to document the changes to RubyInstaller1 for now.

CHANGELOG.md

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+## RubyInstaller1 -> RubyInstaller2
+
+- Built with gcc compiler suite 6.3 from the MSYS2 repository.
+- No devkit anymore, but integrates MSYS2 libraries and toolchain.
+- `Encoding.default_external` is now `UTF-8` (no longer ancient cpXYZ encoding when reading files).
+- RubyInstaller2 uses a separate DLL directory to avoid conflicting DLLs in the PATH.
+- RubyInstaller2 uses a DLL loading mechanism which ignores the PATH environment variable for DLL lookups, but provides a API for DLL directory addition.
+- RubyInstaller2 bundles it's own SSL/TLS CA list derived from the current Mozilla CA list.
+- Ability to add use rubyinstaller-build.gem for customized Ruby-Applications.
+- CI tests on appveyor for each git commit.