doc: add takeoff list

doc/preflight_checklist.md

+ChangeLog
+-----------------------
+* Update NEWS.md
+* Update README.md
+* Commit and push
+
+FFmpeg
+-----------------------
+* Build and test iOS and Android demo locally
+* Modify ffmpeg version in init-ios.sh and init-android.sh
+* Modify ffmpeg version in `IJKFFMoviePlayerController` (by running `sh init-ios.sh`)
+* Commit and push
+
+ijkplayer
+-----------------------
+* Update version.sh
+* Create a tag (subtitle)
+* Commit and push (TAG ONLY)
+
+Travis-ci
+-----------------------
+* Modify ijk version in `.travis.yaml` in iOS and Android ci repo.
+* Ensure compile task has been started on travis-ci.
+* Ensure Andoird release has been released in bintray.
+
+Take off
+-----------------------
+* Push master to github