Minor improvements in README

Signed-off-by: Clifford Wolf <clifford@clifford.at>

README.md

@@ -34,11 +34,24 @@ compatible license that is similar in terms to the MIT license
 or the 2-clause BSD license).
 
 
-Web Site
-========
+Web Site and Other Resources
+============================
 
 More information and documentation can be found on the Yosys web site:
-http://www.clifford.at/yosys/
+- http://www.clifford.at/yosys/
+
+The "Documentation" page on the web site contains links to more resources,
+including a manual that even describes some of the Yosys internals:
+- http://www.clifford.at/yosys/documentation.html
+
+The file `CodingReadme` in this directory contains additional information
+for people interested in using the Yosys C++ APIs.
+
+Users interested in formal verification might want to use the formal verification
+front-end for Yosys, SymbiYosys:
+- https://symbiyosys.readthedocs.io/en/latest/
+- https://github.com/YosysHQ/SymbiYosys
+
 
 Setup
 ======