Add changelog for the esp8266 core

changes.md

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+
+# Current version
+
+- Add 32-bit Linux toolchain
+- Better connection handling in ESP8266WebServer.
+  The server now sends Content-Length and Connection: close headers,
+  then waits for the client to disconnect. By not closing the connection
+  actively, server avoids TIME_WAIT TCP state, and the TCP stack is able to 
+  release the memory immediately, without waiting for 2xMSL period. 
+  If the client doesn't disconnect in 2000ms, the server closes the connection
+  actively.
+- Add Hash library, which has a function to calculate SHA1 hash.
+
+---
+
+# 1.6.4-g545ffde
+19 May, 2015
+
+- Initial release of board manager package
+