Document AHC versioning and add more 2.1 API changes, close AsyncHttpClient#1501

Motivation:

Users might expect SEMVER and would get suprised when upgrading.

Modification:

* Explain AHC versioning in README.md
* Document some AHC config changes that were missing from CHANGES.md

Result:

Version explained

Author: slandelle

CHANGES.md

@@ -7,4 +7,11 @@
 * `AsyncHttpClient` now exposes stats with `getClientStats`.
 * `AsyncHandlerExtensions` was dropped in favor of default methods in `AsyncHandler`.
 * `WebSocket` and `WebSocketListener` methods were renamed to mention frames
-* `AsyncHttpClientConfig#isAggregateWebSocketFrameFragments` now lets you disable WebSocket fragmented frames aggregation
+* `AsyncHttpClientConfig` various changes:
+  * new `getCookieStore` now lets you configure a CookieStore (enabled by default)
+  * new `isAggregateWebSocketFrameFragments` now lets you disable WebSocket fragmented frames aggregation
+  * new `isUseLaxCookieEncoder` lets you loosen cookie chars validation
+  * `isAcceptAnyCertificate` was dropped, as it didn't do what its name stated
+  * new `isUseInsecureTrustManager` lets you use a permissive TrustManager, that would typically let you accept self-signed certificates
+  * new `isDisableHttpsEndpointIdentificationAlgorithm` disables setting `HTTPS` algorithm on the SSLEngines, typically disables SNI and HTTPS hostname verification
+  * new `isAggregateWebSocketFrameFragments` lets you disable fragmented WebSocket frames aggregation

README.md

@@ -19,6 +19,16 @@ Binaries are deployed on Maven central:
 </dependency>
 ```
 
+## Version
+
+AHC doesn't use SEMVER, and won't.
+
+* MAJOR = huge refactoring
+* MINOR = new features and minor API changes, upgrading should require 1 hour of work to adapt sources
+* FIX = no API change, just bug fixes, only those are source and binary compatible with same minor version
+
+Check CHANGES.md for migration path between versions.
+
 ## Basics
 
 Feel free to check the [Javadoc](http://www.javadoc.io/doc/org.asynchttpclient/async-http-client/) or the code for more information.