Initial version of CHANGELOG (ARMmbed#2497)

- Add initial version of CHANGELOG.md
- Use github template

.github/pull_request_template.md

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+<!--
+Please, update the CHANGELOG.md if making major change.
+-->
+
+### Summary of changes
+
+Hopefully your commit message answers to these questions?
+
+* why did you make this change?
+* what did you change?
+* Explain how this change affetcs to the functionality
+
+
+### Pull request type
+
+    [] Feature
+    [] Change
+    [] Bug-fix
+
+
+### Reviewers
+
+<!--@username or @team -->
+@

CHANGELOG.md

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+# Changelog for Nanostack
+
+## Release vxx.x.x
+
+### Features
+* 
+
+### Changes
+* 
+
+### Bugfix
+* 
+
+
+## Release v12.6.2 (09-11-2020)
+
+### Features
+* Allow nanostack max trace level configuration
+* MPL parameters configured based on network size
+* Added faster recovery based on Broadcast schedule id change detection
+* Updated EAPOL relay target when attaching to the network
+
+### Changes
+* Tracing improvements
+
+### Bugfix
+* If DAO is retried re-register address to parent with ARO to confirm downward route
+* socket reference counter possible overflow with big heap sizes
+* Prevent RPL to send DIO when DAO process is ongoing
+
+## Release v12.6.1 (29-10-2020)
+* Start collecting change log.