docs: issue/pr templates, commit message convention

Author: DongWoo Kim

docs/COMMIT_MESSAGE_CONVENTION.md

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+# Commit Message Convention
+
+## Commit Message Format
+
+```
+<Type>: Short description (fix #1234)
+
+Logger description here if necessary
+
+BREAKING CHANGE: only contain breaking change
+```
+* Any line of the commit message cannot be longer 100 characters!
+
+## Revert
+```
+revert: commit <short-hash>
+
+This reverts commit <full-hash>
+More description if needed
+```
+
+## Type
+Must be one of the following:
+
+* **feat**: A new feature
+* **fix**: A bug fix
+* **docs**: Documentation only changes
+* **style**: Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc)
+* **refactor**: A code change that neither fixes a bug nor adds a feature
+* **perf**: A code change that improves performance
+* **test**: Adding missing or correcting existing tests
+* **chore**: Changes to the build process or auxiliary tools and libraries such as documentation generation
+
+## Subject
+* use the imperative, __present__ tense: "change" not "changed" nor "changes"
+* don't capitalize the first letter
+* no dot (.) at the end
+* reference GitHub issues at the end. If the commit doesn’t completely fix the issue, then use `(refs #1234)` instead of `(fixes #1234)`.
+
+## Body
+
+* use the imperative, __present__ tense: "change" not "changed" nor "changes".
+* the motivation for the change and contrast this with previous behavior.
+
+## BREAKING CHANGE
+* This commit contains breaking change(s).
+* start with the word BREAKING CHANGE: with a space or two newlines. The rest of the commit message is then used for this.
+
+This convention is based on [AngularJS](https://github.com/angular/angular.js/blob/master/DEVELOPERS.md#commits) and [ESLint](https://eslint.org/docs/developer-guide/contributing/pull-requests#step2)

docs/ISSUE_TEMPLATE.md

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+<!--
+Thank you for your contribution.
+
+When writing an issue, please, use the template below.
+It's mandatory to use the template for submitting the new issue.
+We don't reply to the issue not following the template.
+-->
+
+<!-- BUG ISSUE TEMPLATE -->
+## Version
+<!-- Write the version of the toast-ui.vue-grid you are currently using. -->
+
+## Test Environment
+<!-- Write the browser type, OS and so on -->
+
+## Current Behavior
+<!-- Write steps to reproduce the current behaviour in detail.
+You can add sample code, 'CodePen' or 'jsfiddle' links. -->
+
+```js
+// Write example code
+```
+
+## Expected Behavior
+<!-- Write a description for future action. -->

docs/PULL_REQUEST_TEMPLATE.md

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+<!-- EDIT TITLE PLEASE -->
+<!-- It should be one of them
+  <ISSUE TYPE>: Short Description (<CLOSING TYPE> #<ISSUE NUMBERS>)
+  ex)
+  feat: add new feature (close #111)
+  fix: wrong behavior (fix #111)
+  chore: change build tool (ref #111)
+-->
+
+<!-- SPECIFY A ISSUE TYPE AT HEAD
+  feat: A new feature
+  fix: A bug fix
+  docs: Documentation only changes
+  style: Changes that do not affect the meaning of the code (white-space, formatting etc)
+  refactor: A code change that neither fixes a bug or adds a feature
+  perf: A code change that improves performance
+  test: Adding missing tests
+  chore: Changes to the build process or auxiliary tools and libraries such as documentation generation
+-->
+
+<!-- ADD CLOSING TYPE AND ISSUE NUMBER AT TAIL
+  (<CLOSING TYPE> #<ISSUE NUMBERS>)
+  close: resolve not a bug(feature, docs, etc) completely
+  fix: resolve a bug completely
+  ref: not fully resolved or related to
+-->
+
+### Please check if the PR fulfills these requirements
+- [ ] It's the right issue type on the title
+- [ ] When resolving a specific issue, it's referenced in the PR's title (e.g. `fix #xxx[,#xxx]`, where "xxx" is the issue number)
+- [ ] The commit message follows our guidelines
+- [ ] Tests for the changes have been added (for bug fixes/features)
+- [ ] Docs have been added/updated (for bug fixes/features)
+- [ ] It does not introduce a breaking change or has a description of the breaking change
+
+### Description
+
+
+
+---
+Thank you for your contribution to TOAST UI product. 🎉 😘 ✨