README should mention DSLX and cc frontend more prominently

[proppy]

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