2.6.1 release process

[d-a-v]

Reference

[X] Open a new issue to track activities, which will be closed after the release is done.
[X] Assemble release notes: https://demo.firepad.io/#esp8266arduino261
[X] Make a PR with below, wait for Travis CI, and merge

• updated release on platform.txt
• updated cores/esp8266/TZ.h

[X] waiting for other maintainers to check on release notes
[X] Tag the latest commit on the master branch
[X] Push the tag to then push the tag to esp8266/Arduino Github repository
[X] Wait for Travis CI build for the tag to pass, see https://travis-ci.org/esp8266/Arduino/builds/611914385
[X] Check that the new (draft) release has been created (no editing at this point!), see https://github.com/esp8266/Arduino/releases/tag/2.6.1
[X] Check that the boards manager package .zip file has been successfully uploaded as a release artifact.
[X] Check that the package index downloaded from https://arduino.esp8266.com/stable/package_esp8266com_index.json contains an entry for the new version (it may not be the first one).
[X] Navigate to release list in Github here https://github.com/esp8266/Arduino/releases, press "Edit" button to edit release description, paste release notes, and publish it.
[X] In the issue tracker, remove "staged-for-release" label for all issues which have it, and close them. Close the milestone associated with the released version.
[X] Check that https://arduino-esp8266.readthedocs.io/en/latest/ has a new doc build for the new tag, and that "stable" points to that build. If a new build did not trigger, log into readthedoc's home here https://readthedocs.org/ (account must have been added to project as maintainer) and trigger it manually.
[X] Create a commit to the master branch, updating the version in platform.txt file. This should correspond to the version of the next milestone, plus -dev suffix. E.g. 3.1.0-dev. #6774

• In main README.md, in "Latest release" section, change version number in the readthedocs link to the version which was just released, and verify that all links work.