Added notes on compile tests

Author: fuine

tests/compile-fail/README.md

@@ -0,0 +1,22 @@
+This directory contains snippets of code that should yield a
+warning/note/help/error at compilation. Syntax of annotations is described in
+[rust documentation](https://github.com/rust-lang/rust/blob/master/src/test/COMPILER_TESTS.md).
+For more information check out [`compiletest` crate](https://github.com/laumann/compiletest-rs).
+
+To run compile tests issue `cargo +nightly --test --features compiletest`.
+
+## Notes on working with `compiletest` crate
+
+* Currently code that is inside macro should not be annotated, as `compiletest`
+    crate cannot deal with the fact that macro invocations effectively changes
+    line numbering. To prevent this add a `// error-pattern:<your error message here>`
+    on the top of the file and make sure that you set `deny` lint level
+    if you want to test compiler message different than error.
+* `compiletest` crate by default sets `allow(dead_code)` lint level so make sure
+    that you change it to something suiting your needs even if the warning is
+    issued prior to any macro invocation.
+* If you get a message `error: 0 unexpected errors found, 1 expected errors not found`
+  despite the fact that some error was bound to occur don't worry - it's a known
+  issue in the `compiletest` crate and your error was probably not registered -
+  make sure that your annotations are correct and that you are setting correct
+  lint levels.