[MRG] Add logo to POT (#357)

* add logo code and logo to doc

* update release file

Author: rflamary

RELEASES.md

@@ -5,6 +5,7 @@
 
 #### New features
 
+- A brand new logo for POT (PR #357)
 - Better list of related examples in quick start guide with `minigallery` (PR #334).
 - Add optional log-domain Sinkhorn implementation in WDA to support smaller values
   of the regularization parameter (PR #336).