README should mention DSLX and cc frontend more prominently

I'd great if the intro of the README:
```
XLS implements a High Level Synthesis (HLS) toolchain which produces synthesizable designs from flexible, high-level descriptions of functionality.
```
was mentioning the different frontend available to express the "high-level descriptions of functionality":
- DSLX: Rust-sh; immutable expression; dataflow; hardware-oriented features
- xlscc: familiar C++ syntax; experimental; hardware-oriented annotations