Add Apache 2.0 License.

Signed-off-by: HE, Tao <sighingnow@gmail.com>

LICENSE

@@ -0,0 +1,13 @@
+Copyright 2019 Tao He
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.

README.md

@@ -25,5 +25,11 @@ This theme can be used just as other [Jekyll themes][1].
 
 Fork this repository and add your markdown posts to the `_posts` folder.
 
+## License
+
+This work is open sourced under the Apache License, Version 2.0.
+
+Copyright 2019 Tao He.
+
 [2]: https://pages.github.com
 [2]: https://pages.github.com/themes

_posts/2019-04-29-license.md

@@ -0,0 +1,12 @@
+---
+title: License
+author: Tao He
+date: 2019-04-28
+category: Jekyll
+layout: post
+---
+
+This work is open sourced under the Apache License, Version 2.0, using the
+same license as the original [GitBook](https://github.com/GitbookIO/gitbook) repository.
+
+Copyright 2019 Tao He.