feat: add docs warning

Author: gadomski

docs/index.md

@@ -1,8 +1,24 @@
 # PySTAC
 
+!!! warning
+
+    These docs are for the work-in-progress v2 of PySTAC.
+    For the current PySTAC v1 docs, see <https://pystac.readthedocs.io>.
+
+    Our work plan for v2 goes like this:
+
+    1. Rebuild the core data structures (`Item`, `Catalog`, `Collection`, etc) from scratch, with new tests
+    2. Slowly re-add the old tests to the `tests/v1` one at a time, to make sure that we're breaking as little as possible
+    3. If we intentionally break a test (e.g. by relaxing a check on inputs) we'll mark it `xfail` and copy it to test the new expected behavior
+
+    This will take a while.
+    Watch <https://github.com/stac-utils/pystac/tree/v2> to track our progress.
+    We'll sometimes use pull requests, but sometimes not.
+
 **PySTAC** is a Python library for reading and writing [SpatioTemporal Asset Catalog (STAC)](https://stacspec.org) metadata.
 To install:
 
+<!-- markdownlint-disable MD046 -->
 ```shell
 python -m pip install pystac
 ```