Add basic UI test docs on legacy compiletest-style headers vs ui_test…

…-style headers

tests/ui/README.md

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+# UI Tests
+
+This folder contains `rustc`'s
+[UI tests](https://rustc-dev-guide.rust-lang.org/tests/ui.html).
+
+## Test Directives (Headers)
+
+Typically, a UI test will have some test directives / headers which are
+special comments that tell compiletest how to build and intepret a test.
+
+As part of an on-going effort to rewrite compiletest
+(see <https://github.com/rust-lang/compiler-team/issues/536>), a major
+change proposal to change legacy compiletest-style headers `// <directive>`
+to [`ui_test`](https://github.com/oli-obk/ui_test)-style headers
+`//@ <directive>` was accepted (see
+<https://github.com/rust-lang/compiler-team/issues/512>.
+
+An example directive is `ignore-test`. In legacy compiletest style, the header
+would be written as
+
+```rs
+// ignore-test
+```
+
+but in `ui_test` style, the header would be written as
+
+```rs
+//@ ignore-test
+```
+
+compiletest is changed to accept only `//@` directives for UI tests
+(currently), and will reject and report an error if it encounters any
+comments `// <content>` that may be parsed as an legacy compiletest-style
+test header. To fix this, you should migrate to the `ui_test`-style header
+`//@ <content>`.