Add README for fed2020 and update main README.

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+# ustaxlib/fed2020
+
+Forms for US tax year 2020.
+
+This is an *overlay* of the [fed2019](../fed2019/README.md) module. An overlay module inherits (and
+therefore supports) all of the forms from its parent.
+
+# Adjustments
+Besides updating the tax table and inflation-adjusted constants of the **Tax Return**, updated
+versions of the following forms are provided. These replace the forms from the fed2019 overlay:
+
+- **Form 1040**
+- **Schedule 3**