Add AR to README

Author: m-mohr

README.md

@@ -48,6 +48,9 @@ Below you can find all Product Family Specifications (PFS) that are currently de
   - **Surface Temperature (ST)**
 
     *Latest draft version: [HTML](https://ceos-org.github.io/ceos-ard/latest/ST.html), [PDF](https://ceos-org.github.io/ceos-ard/latest/ST.pdf), [Word (docx)](https://ceos-org.github.io/ceos-ard/latest/ST.docx)*
+  - **Aquatic Reflectance (AR)**
+  
+    *Latest draft version: [HTML](https://ceos-org.github.io/ceos-ard/latest/AR.html), [PDF](https://ceos-org.github.io/ceos-ard/latest/AR.pdf), [Word (docx)](https://ceos-org.github.io/ceos-ard/latest/AR.docx)*
   - **Nighttime Lights Surface Radiance (NLSR)**
 
     *Latest draft version: [HTML](https://ceos-org.github.io/ceos-ard/latest/NLSR.html), [PDF](https://ceos-org.github.io/ceos-ard/latest/NLSR.pdf), [Word (docx)](https://ceos-org.github.io/ceos-ard/latest/NLSR.docx)*