Add rustc UX guidelines.

See https://github.com/rust-lang/rfcs/pull/1246 for discussion.

src/doc/rustc-ux-guidelines.md

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+Don't forget the user. Whether human or another program, such as an IDE, a
+good user experience with the compiler goes a long way into making developer
+lives better. We don't want users to be baffled by compiler output or
+learn arcane patterns to compile their program.
+
+## Error, Warning, Help, Note Messages
+
+When the compiler detects a problem, it can emit either an error, warning,
+note, or help message.
+
+An `error` is emitted when the compiler detects a problem that makes it unable
+ to compile the program, either because the program is invalid or the
+ programmer has decided to make a specific `warning` into an error.
+
+A `warning` is emitted when the compiler detects something odd about a
+program. For instance, dead code and unused `Result` values.
+
+A `help` is emitted following either an `error` or `warning` giving extra
+information to the user about how to solve their problem.
+
+A `note` is for identifying additional circumstances and parts of the code
+that lead to a warning or error. For example, the borrow checker will note any
+previous conflicting borrows.
+
+* Write in plain simple English. If your message, when shown on a – possibly
+small – screen (which hasn't been cleaned for a while), cannot be understood
+by a normal programmer, who just came out of bed after a night partying, it's
+too complex.
+* `Errors` and `Warnings` should not suggest how to fix the problem. A `Help`
+message should be emitted instead.
+* `Error`, `Warning`, `Note`, and `Help` messages start with a lowercase
+letter and do not end with punctuation.
+* Error messages should be succinct. Users will see these error messages many
+times, and more verbose descriptions can be viewed with the `--explain` flag.
+That said, don't make it so terse that it's hard to understand.
+* The word "illegal" is illegal. Prefer "invalid" or a more specific word
+instead.
+* Errors should document the span of code where they occur – the `span_..`
+methods allow to easily do this. Also `note` other spans that have contributed
+to the error if the span isn't too large.
+* When emitting a message with span, try to reduce the span to the smallest
+amount possible that still signifies the issue
+* Try not to emit multiple error messages for the same error. This may require
+detecting duplicates.
+* When the compiler has too little information for a specific error message,
+lobby for annotations for library code that allow adding more. For example see
+`#[on_unimplemented]`. Use these annotations when available!
+* Keep in mind that Rust's learning curve is rather steep, and that the
+compiler messages are an important learning tool.
+
+## Error Explanations
+
+Error explanations are long form descriptions of error messages provided with
+the compiler. They are accessible via the `--explain` flag. Each explanation
+comes with an example of how to trigger it and advice on how to fix it.
+
+* All of them are accessible [online](https://github.com/rust-lang/rust/blob/master/src/librustc/diagnostics.rs).
+* Explanations have full markdown support. Use it, especially to highlight
+code with backticks.
+* When talking about the compiler, call it `the compiler`, not `Rust` or
+`rustc`.
+
+## Compiler Flags
+
+* Flags should be orthogonal to each other. For example, if we'd have a
+json-emitting variant of multiple actions `foo` and `bar`, an additional
+--json flag is better than adding `--foo-json` and `--bar-json`.
+* Always give options a long descriptive name, if only for better
+understandable compiler scripts.
+* The `--verbose` flag is for adding verbose information to `rustc` output
+when not compiling a program. For example, using it with the `--version` flag
+gives information about the hashes of the code.
+* Experimental flags and options must be guarded behind the `-Z unstable-options` flag.