Add a section to readme about limitations

[brson]

• It depends on Rust 1.48+
• It depends on a c++17 compiler, and should emit a readable error if one is not available.
• The wasm-opt-sys crate takes a non-negligible amount of time to build (minutes on my underpowered laptop). It also does not do any incremental recompilation, so if the build is invalidated it will rebuild the C++ code from scratch. The lack of incremental recompilation is a limitation self-imposed by not using cmake or other external build system.
• wasm-opt on windows does not support extended unicode paths (probably anything non-ascii, maybe latin-1, it's unclear). This is a limitation of binaryen and not a regression of the bindings. It may or may not be fixed in the future. The APIs will return an error if this occurs.
• cargo tarpaulin (code coverage) segfaults running any wasm-opt crates (Investigate tarpaulin segfault #59), reason unknown. This behavior could infect other crates that link to wasm-opt. If you use tarpaulin, you might check it.