Add 1.8.0 relnotes to RELEASE.md
Add 1.8.0 relnotes to RELEASE.md

tensorflow/tensorboard,qiuminxu/tensorboard,qiuminxu/tensorboard,qiuminxu/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,qiuminxu/tensorboard,tensorflow/tensorboard,qiuminxu/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,qiuminxu/tensorboard,tensorflow/tensorboard