Update README to consider custom scalars plugin (#951)

The custom scalars plugin lets the user overlap custom run-tag pairs
within the same plot as well as visualize margins. This change modifies
the main TensorBoard README to note those new features.

Author: chihuahua

README.md

@@ -325,11 +325,18 @@ If you need access to the full dataset, you can read the event files that
 TensorBoard consumes by using the [`summary_iterator`](https://github.com/tensorflow/tensorflow/blob/e7f333b5f8b3c53b21d149d8d14c0cebbde431aa/tensorflow/python/summary/summary_iterator.py#L313)
 method.
 
+### Can I customize which lines appear in a plot?
 
-### Can I overlap multiple plots?
+Using the [custom scalars plugin](tensorboard/plugins/custom_scalar), you can
+create scalar plots with lines for custom run-tag pairs. However, within the
+original scalars dashboard, each scalar plot corresponds to data for a specific
+tag and contains lines for each run that includes that tag.
 
-Right now, you can overlap plots only if they are from different runs, and both
-have the same tag name.
+### Can I visualize margins above and below lines?
+
+Margin plots (that visualize lower and upper bounds) may be created with the
+[custom scalars plugin](tensorboard/plugins/custom_scalar). The original
+scalars plugin does not support visualizing margins.
 
 ### Can I create scatterplots (or other custom plots)?