Add Captum Insights deprecation message to README (#1498)

Summary:
Pull Request resolved: #1498

Add a message about deprecating Captum Insights in an upcoming release. We intend to deprecate it fully but still provide the code for those interested in it.

Reviewed By: cyrjano

Differential Revision: D69695277

fbshipit-source-id: d66ffa9db91dfd0025f286b72fe8aeff62ad5a0b

Author: craymichael

README.md

@@ -90,6 +90,7 @@ pip install -e .
 To customize the installation, you can also run the following variants of the
 above:
 * `pip install -e .[insights]`: Also installs all packages necessary for running Captum Insights.
+**NOTE**: Captum Insights is being deprecated. See further details [below](#captum-insights).
 * `pip install -e .[dev]`: Also installs all tools necessary for development
   (testing, linting, docs building; see [Contributing](#contributing) below).
 * `pip install -e .[tutorials]`: Also installs all packages necessary for running the tutorial notebooks.
@@ -386,6 +387,10 @@ Captum on different types of models can be found in our tutorials.
 
 ## Captum Insights
 
+**NOTE**: *Support for Captum Insights is being deprecated in an upcoming release.
+While the code will still be available, there will no longer be active
+development or support for it.*
+
 Captum provides a web interface called Insights for easy visualization and
 access to a number of our interpretability algorithms.