Recommend to use c++filt (#755)

Since 2016-11-16 c++filt [1] recognizes rust legacy demangling and will
supports newer one(v0 name mangling) [2] [3], so it would be better choice.

[1]: https://sourceware.org/binutils/docs/binutils/c_002b_002bfilt.html
[2]: rust-lang/rfcs#2603
[3]: https://sourceware.org/bugzilla/show_bug.cgi?id=27194

Author: kubo39

README.md

@@ -148,14 +148,16 @@ $ cargo test
 
 Prerequisites:
 
-- Install [`perf`](https://perf.wiki.kernel.org/index.php/Main_Page), [`stackcollapse-perf`](https://github.com/brendangregg/FlameGraph/blob/master/flamegraph.pl), [`rust-unmangle`](https://github.com/Yamakaky/rust-unmangle/blob/master/rust-unmangle) and [`flamegraph`](https://github.com/brendangregg/FlameGraph/blob/master/flamegraph.pl)
+- Install [`perf`](https://perf.wiki.kernel.org/index.php/Main_Page), [`stackcollapse-perf`](https://github.com/brendangregg/FlameGraph/blob/master/flamegraph.pl), [`c++filt`](https://sourceware.org/binutils/docs/binutils/c_002b_002bfilt.html) and [`flamegraph`](https://github.com/brendangregg/FlameGraph/blob/master/flamegraph.pl)
 
 ```shell
 $ RUSTFLAGS='-g' cargo build --release --all-targets # build target
 $ sudo perf record --call-graph dwarf ./target/release/examples/dlint benchmarks/oak/**.ts # create performance profile
-$ perf script | stackcollapse-perf | rust-unmangle | flamegraph > flame.svg # generate flamegraph
+$ perf script | stackcollapse-perf | c++filt | flamegraph > flame.svg # generate flamegraph
 ```
 
+You can use [rust-unmangle](https://github.com/Yamakaky/rust-unmangle/blob/master/rust-unmangle) or [rustfilt](https://github.com/luser/rustfilt) instead of c++filt.
+
 These commands can take a few minutes to run.
 
 ## Contributing