Add 1.5.1 relnotes to RELEASE.md (#956)

Cherrypick to add 1.5.1 relnotes back to master.

Author: nfelt

RELEASE.md

@@ -3,6 +3,31 @@
 The 1.6 minor series tracks TensorFlow 1.6. Release notes upcoming.
 
 
+# Release 1.5.1
+
+NOTICE: TensorBoard 1.6.0+ will move to the `tensorboard` package name on PyPI,
+instead of using `tensorflow-tensorboard`. Only bugfix updates on 1.5.x will be
+applied to the old package name. To upgrade to TensorBoard 1.6.0+ please *first*
+`pip uninstall tensorflow-tensorboard` before doing `pip install tensorboard`.
+
+The 1.5 minor series tracks TensorFlow 1.5.
+
+## Bug fixes
+
+- #554 - line charts no longer "shrink" after run changes on other tabs
+- #889 - fixed xComponentsCreationMethod default in vz-line-chart
+- #898 - fixed offset of checkbox label in projector dashboard - thanks @marcj
+- #903 - disabled margin plot smoothing to avoid line going out of bounds
+- #916 - made `futures` dependency py2-only to avoid install failures
+- #924 - fixed graph dashboard bug causing blank PNG download and minimap
+- #926 - made projector plugin API available in pip package
+
+## Documentation updates
+
+- Custom scalars documentation now documents margin plots feature (#878)
+- FAQ updated to describe custom scalars plugin use cases
+
+
 # Release 1.5.0
 
 The 1.5 minor series tracks TensorFlow 1.5.