credits: update license of axum

Signed-off-by: Yu Li <liyu.yukiteru@bytedance.com>

CREDITS.md

@@ -18,3 +18,10 @@ We've copied and modified some code of [`tonic`](https://github.com/hyperium/ton
 We really appreciate the work that the `tonic` team have done.
 
 `Tonic` is licensed under the MIT license, and a copy can be found under the `licenses/tonic` directory.
+
+# axum
+
+We've copied and modified some code of [`axum`](https://github.com/tokio-rs/axum).
+We really appreciate the work that the `axum` team have done.
+
+`axum` is licensed under the MIT license, and a copy can be found under the `licenses/axum` directory.

licenses/axum/LICENSE

@@ -0,0 +1,25 @@
+Copyright (c) 2019 Axum Contributors
+
+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.