updated docs and pull request template

docs/code_style.md

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+# Boostnote Code Style
+When submitting your PR, please make sure that your code is well tested and follow the code style of Boostnote.
+
+The code style of Boostnote is listed in [`.eslintrc`](.eslintrc). We also have additional code styles that is not mentioned in that file.
+
+## Additional code styles
+### Use ES6 Object Destructing
+
+Please use Object Destructing whenever it's possible.
+
+**Example**: Importing from library
+
+```js
+
+// BAD
+import Code from 'library'
+const subCode = Code.subCode
+subCode()
+
+// GOOD
+import { subCode } from 'library'
+subCode()
+```
+
+**Example**: Extract data from react component state
+
+```
+// BAD
+<h1>{this.state.name}</h1>
+
+// GOOD
+const { name } = this.state
+<h1>{name}</h1>
+```
+
+### Use meaningful name
+This is actually not a "code style" but rather a requirement in every projects. Please name your variables carefully.
+
+**Example**: Naming callback function for event
+
+```js
+// BAD
+<h1 onclick={onClick}>Name</h1>
+
+// GOOD
+<h1 onclick={onNameClick}>Name</h1>
+```
+
+### Don't write long conditional statement
+When writing a conditional statement, if it's too long, cut it into small meaningful variables.
+
+```js
+// BAD
+if (note.type == 'markdown' && note.index == 2 && note.content.indexOf('string') != -1)
+
+// GOOD
+const isSecondMarkdownNote = note.type == 'markdown' && note.index == 2
+const isNoteHasString = note.content.indexOf('string') != -1
+if (isSecondMarkdownNote && isNoteHasString)
+```