counter: add descriptive README.md

basics/counter/README.md

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+# Counter
+
+This example program allows anyone to create a counter and increment it.
+
+Any counter can be incremented by any key.
+
+## Note: Seahorse
+
+Seahorse currently does not allow the program to initialize anchor 
+accounts unless they are PDAs. 
+
+Seahorse example only allows users to increment the counter that corresponds to their public key.
+
+