Make CC BY 4.0 on the licensing page links to the license

Author: davewalker5

license.md

@@ -67,15 +67,15 @@ These are provided to make reuse straightforward.
 
 ### General content
 
-> Walker, David. Field Notes Journal. CC BY 4.0.
+> Walker, David. Field Notes Journal. [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).
 
 ### Image
 
-> Photo: David Walker, Field Notes Journal, CC BY 4.0
+> Photo: David Walker, Field Notes Journal, [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/)
 
 Example for a modified image:
 
-> Photo: David Walker, Field Notes Journal, CC BY 4.0. Cropped and adjusted.
+> Photo: David Walker, Field Notes Journal, [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/). Cropped and adjusted.
 
 ### Dataset