Document MUXF8 Vivado compatibility issue

Signed-off-by: John McMaster <johndmcmaster@gmail.com>

README.md

@@ -7,6 +7,12 @@ bit-stream format of Xilinx 7-series FPGAs.
 
 # Quickstart Guide
 
+Install Vivado 2017.2 (2017.3 has a known compatibility issue, see
+https://github.com/SymbiFlow/prjxray/issues/14). Then source the settings
+script, ie
+
+    source /opt/Xilinx/Vivado/2017.2/settings64.sh
+
 Pull submodules:
 
     git submodule update --init --recursive

fuzzers/012-clbn5ffmux/generate.sh

@@ -1,6 +1,7 @@
 #!/bin/bash
 
 set -ex
+if [ $(vivado -h |grep Vivado |cut -d\  -f 2) != "v2017.2" ] ; then echo "FIXME: requires Vivado 2017.2. See https://github.com/SymbiFlow/prjxray/issues/14"; exit 1; fi
 
 source ${XRAY_GENHEADER}
 

fuzzers/015-clbnffmux/generate.sh

@@ -1,6 +1,7 @@
 #!/bin/bash
 
 set -ex
+if [ $(vivado -h |grep Vivado |cut -d\  -f 2) != "v2017.2" ] ; then echo "FIXME: requires Vivado 2017.2. See https://github.com/SymbiFlow/prjxray/issues/14"; exit 1; fi
 
 source ${XRAY_GENHEADER}
 

fuzzers/016-clbnoutmux/generate.sh

@@ -1,6 +1,7 @@
 #!/bin/bash
 
 set -ex
+if [ $(vivado -h |grep Vivado |cut -d\  -f 2) != "v2017.2" ] ; then echo "FIXME: requires Vivado 2017.2. See https://github.com/SymbiFlow/prjxray/issues/14"; exit 1; fi
 
 source ${XRAY_GENHEADER}