LICENCE-MIT: Add the MIT text

Generated with:

  $ curl -s https://raw.githubusercontent.com/rust-lang/rust/1.23.0/LICENSE-MIT >LICENSE-MIT

Rust dropped their copyright line from LICENSE-MIT in
rust-lang/rust@2a8807e8 (LICENSE-MIT: Remove inaccurate
(misattributed) copyright notice, 2017-07-26, [1]).  I'm not clear on
how that interacts with the "The above copyright notice and this
permission notice shall be included..." line [2], but it's better than
not including the MIT text at all (which is where we were before this
commit).

[3] has the same text (although it has a copyright template), but
using Rust as the source gets us the Rust-standard line wrappings ;).

[1]: rust-lang/rust@2a8807e
[2]: rust-lang/rust#43498 (comment)
[3]: https://opensource.org/licenses/MIT

LICENSE-MIT

@@ -0,0 +1,23 @@
+Permission is hereby granted, free of charge, to any
+person obtaining a copy of this software and associated
+documentation files (the "Software"), to deal in the
+Software without restriction, including without
+limitation the rights to use, copy, modify, merge,
+publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software
+is furnished to do so, subject to the following
+conditions:
+
+The above copyright notice and this permission notice
+shall be included in all copies or substantial portions
+of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
+ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
+TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
+SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
+IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.