Add links to docs to readme (#800)

Author: tcmetzger

README.md

@@ -50,6 +50,11 @@
          alt="Current github actions build status" />
     </a>
     </div>
+    <div>
+      <a href='https://xarray-spatial.readthedocs.io/en/latest/?badge=latest'>
+        <img src='https://readthedocs.org/projects/xarray-spatial/badge/?version=latest' alt='Documentation Status' />
+      </a>
+    </div>
   </td>
   <td>Coverage</td>
   <td>
@@ -120,6 +125,7 @@ In all the above, the command will download and store the files into your curren
 
 `xarray-spatial` does not depend on GDAL / GEOS, which makes it fully extensible in Python but does limit the breadth of operations that can be covered.  xarray-spatial is meant to include the core raster-analysis functions needed for GIS developers / analysts, implemented independently of the non-Python geo stack.
 
+Our documentation is still under construction, but [docs can be found here](https://xarray-spatial.readthedocs.io/en/latest/).
 
 #### Raster-huh?