Dual license under MIT and Apache (#121)

* Dual license under MIT and Apache

* Fix links

* Fix license notice in documentation

* Update again

* Revert "Update again"

This reverts commit 0596f5e.

LICENSE-MIT

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) Yota Toyama
+
+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.

LICENSE.md

@@ -0,0 +1,3 @@
+# License
+
+Pen is dual-licensed under [MIT](LICENSE-MIT) and [Apache 2.0](LICENSE-APACHE).

README.md

@@ -4,4 +4,4 @@ This is a pen.
 
 ## License
 
-[Apache 2.0](LICENSE)
+Dual-licensed under [MIT](LICENSE-MIT) and [Apache 2.0](LICENSE-APACHE).

doc/src/README.md

@@ -67,4 +67,4 @@ The runtime and library provide tools for thread-safe parallel computation that
 
 ## License
 
-[Apache 2.0](https://github.com/pen-lang/pen/blob/main/LICENSE)
+Dual-licensed under [MIT](https://github.com/pen-lang/pen/blob/main/LICENSE-MIT) and [Apache 2.0](https://github.com/pen-lang/pen/blob/main/LICENSE-APACHE).