Add compatibility notes to the documentation

CHANGELOG.md

@@ -28,10 +28,16 @@ Prevent the CPU overutilization when waiting for the process to exit.
 
 Non-code changes:
 * Documentation refactoring and update
-* Introduced the CI flow being hosted on Appveyor. The project page is [here](https://ci.appveyor.com/project/oleg-nenashev/winsw)
+* Introduced the CI flow being hosted on AppVeyor. The project page is [here](https://ci.appveyor.com/project/oleg-nenashev/winsw)
 * Starting from WinSW 2.0, [GitHub](https://github.com/kohsuke/winsw/releases) and NuGet will be the release sources
  * [Maven repository](http://repo.jenkins-ci.org/releases/com/sun/winsw/winsw/) will be periodically updated on-demand
 
+Compatibility notes:
+* WinSW `2.x` is **fully compatible** with WinSW `1.x` in terms of the command-line interface and configuration files.
+* Any behavior difference will be considered as a bug
+* New features like [WinSW extensions](doc/extensions/extensions.md) are disabled by default. 
+They can be enabled via the configuration file.
+
 ##### 1.19.1
 
 Release date: Nov 05, 2016

README.md

@@ -61,13 +61,16 @@ Naming and download sources for these binaries are currently in flux.
 #### WinSW 2.x
 
 This is a new release line under active development.
-API stability is not guaranteed till the first release, the project structure is in flux.
 
 Major changes since 1.x:
-* Rework of the project structure
-* Better logging
-* [Internal extension engine](doc/extensions/extensions.md), which allows extending the WinSW behavior
-* Executable package targeting the .NET Framework 4.0
+* New executable package targeting the .NET Framework `4.0`. .NET Framework `2.0` is still supported.
+* Migration of the logging subsystem to `Apache log4net`
+* Internal [extension engine](doc/extensions/extensions.md), which allows extending the wrapper's behavior.
+
+See the full changelog in the [release notes](CHANGELOG.md#20).
+
+The version `2.x` is **fully compatible** with the `1.x` configuration file format, 
+  hence the upgrade procedure just requires replacement of the executable file.
 
 #### WinSW 1.x