setup: update pyvcd dependency, for var_type="string".

README.md

@@ -19,6 +19,10 @@ nMigen is designed for Python 3.6 and newer. nMigen's Verilog backend depends on
 [migen]: https://m-labs.hk/migen
 [yosys]: http://www.clifford.at/yosys/
 
+### Installation
+
+    pip install git+https://github.com/m-labs/nmigen.git
+
 ### Introduction
 
 TBD

setup.py

@@ -14,7 +14,7 @@ setup(
     description="Python toolbox for building complex digital hardware",
     #long_description="""TODO""",
     license="BSD",
-    install_requires=["pyvcd", "bitarray"],
+    install_requires=["pyvcd>=0.1.4", "bitarray"],
     packages=find_packages(),
     project_urls={
         #"Documentation": "https://glasgow.readthedocs.io/",