Numfocus (#2409)

* add some blurbs about numfocus sponsorship to docs

* add numfocus to history blurb

* add missing logo file

* markdown to rst in readme

Author: Joe Hamman

README.rst

@@ -109,14 +109,15 @@ NumFOCUS
 --------
 
 .. image:: https://numfocus.org/wp-content/uploads/2017/07/NumFocus_LRG.png
-   :scale: 50 %
+   :scale: 25 %
 
 Xarray is a fiscally sponsored project of NumFOCUS, a nonprofit dedicated
 to supporting the open source scientific computing community. If you like
-Xarray and want to support our mission, please consider making a
-[donation](https://www.flipcause.com/secure/cause_pdetails/MjE3OQ==)
+Xarray and want to support our mission, please consider making a donation_
 to support our efforts.
 
+.. _donation: https://www.flipcause.com/secure/cause_pdetails/MjE3OQ==
+
 History
 -------