Add README.md

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

README.md

+
+
+Running this tests with C++ coverage analysis
+---------------------------------------------
+
+```
+git clone git@github.com:YosysHQ/yosys.git yosys-cover
+cd yosys-cover
+
+# Building Yosys
+make config-gcov
+make -j$(nproc)
+
+# Running built-in tests
+make -j$(nproc) test
+
+# Running yosys-tests
+make -j$(nproc) ystests
+
+# Generate coverage report
+make coverage
+
+# Display coverage data
+xdg-open coverage_html/index.html
+```