Add README.md

Author: nrc

README.md

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+# rls-rustc
+
+A simple shim around rustc to allow using save-analysis with a stable toolchain
+
+## Building and running
+
+`cargo build` or `cargo run`
+
+You probably want to use `--release`
+
+## Support
+
+File an issue or ping nrc in #rust-dev-tools
+
+## Implementation
+
+The compiler has an extensible driver interface. The main API is the `CompilerCalls`
+trait. A tool can emulate the compiler, but adjust operation by implementing
+that trait. This shim does exactly that, using nearly all the defaults, but
+setting some properties that are useful for tools. These are usually only
+available by using a nightly toolchain, but by using this shim, can be used on
+stable.
+
+In the future we might want to make the properties we set configurable.