Add LICENSE and update documents

Author: yuku

LICENSE

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+The MIT License (MIT)
+
+Copyright (c) 2020 Yuku TAKAHASHI
+
+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.

README.md

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 # Textcomplete
 
 > Autocomplete for textarea elements
+
+## Packages
+
+Textcomplete consists of subpackages:
+
+Name                   | Description
+-----------------------|------------------------------
+@textcomplete/core     | Core of Textcomplete.
+@textcomplete/textarea | Editor for Textarea elements.

packages/textcomplete-core/README.md

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 # @textcomplete/core
+
+> Core of Textcomplete

packages/textcomplete-textarea/README.md

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-# textcomplete
+# @textcomplete/textarea
 
-> Autocomplete for Textarea elements.
+> Textcomplete editor implementation for Textarea elements.
+
+## Install
 
 ```bash
-npm add --save textcomplete
+npm install --save @textcomplete/core @texttcomplete/textarea
+```
+
+## Synopsis
+
+```js
+const { Textcomplete } = require("@textcomplete/core")
+const { Textarea } = require("@textcomplete/textarea")
+
+const editor = new Textarea(textareaElement)
+const textcomplete = new Textcomplete(editor, strategies, option)
 ```