[VTA][TSIM] Verilator compile report error for printf (#3438)

[Symptom]
after follow the tsim example readme, doing verilator install by 'sudo apt-get-install verilator'
Once enable 'debug' or manually add 'printf' logic in chisel module, verilator would report
following error.
'syntax error, unexpected INTEGER NUMBER, expecting IDENTIFIER'

[Fix]
upgrade verilator to 4.012, issue fixed.

[Solution]
Link README.md verilator install steps with verilator home website
install instruction.

vta/apps/tsim_example/README.md

@@ -46,6 +46,19 @@ Install `sbt` and `verilator`.
 sudo apt install verilator sbt
 ```
 
+Verilator version check
+
+```bash
+verilator --version
+```
+
+the supported version of Verilator should be at least 4.012, 
+if homebrew (OSX) or package-manager (Linux) does not support that version,
+please install Verilator 4.012 or later from binary or source base on following
+instruction of Verilator wiki.  
+
+https://www.veripool.org/projects/verilator/wiki/Installing
+
 ## Setup in TVM
 
 1. Install `verilator` and `sbt` as described above