docs(contributing): Fix grammar errors

Change numerous grammatical errors on CONTRIBUTING.md without changing
its content

Close #420

AUTHORS.txt

@@ -14,4 +14,5 @@ dtc03012 <dtc03012@naver.com>
 Kim Dong Min <dkrahd12@gmail.com>
 Russell Shingleton <reshingleton@gmail.com>
 Matthias Komarek <matthias@mkomarek.de>
-Tony Quetano <tony.quetano@planttheidea.com>
+Tony Quetano <tony.quetano@planttheidea.com>
+EunChan Park <pec9399@naver.com>

CONTRIBUTING.md

@@ -1,20 +1,20 @@
 # How to contribute to billboard.js
-billboard.js is opened to everyone and we're welcoming for any kind of contribution.
-We believe that our project can grow with your interests helping others' necessities.
+billboard.js is open to everyone, and we welcome any kinds of contribution.
+We believe that our project can grow with your interests in helping others' necessities.
 
 ## Style Guide
 
-billboard.js has several style guidelines to follow.
-Before your start, please read attentively below instructions.
+billboard.js has several style guidelines that you must follow.
+Before your start, please read the below instructions carefully.
 
 ### Linting and Code Convention
-To maintain the code style, we adopted [ESLint](http://eslint.org/) to maintain our code quality. 
-The [rules](https://github.com/naver/eslint-config-naver/tree/master/rules) are modified version based on [Airbnb JavaScript Style Guide](https://github.com/airbnb/javascript). 
+To maintain the code style and quality, we adopted [ESLint](http://eslint.org/). 
+The [rules](https://github.com/naver/eslint-config-naver/tree/master/rules) are based on the [Airbnb JavaScript Style Guide](https://github.com/airbnb/javascript) with some modifications. 
 
 ### Commit Log Guidelines
-billboard.js use formatted commit logs for many different purposes (like creating CHANGELOG, ease history searching, etc.).
-To not break, you'll be forced to follow our commit log guidelines.
-Before your commit/push, make sure following our commit log guidelines.
+billboard.js uses formatted commit logs for many different purposes (like creating CHANGELOG, ease history searching, etc.).
+To not break the current format, you'll be forced to follow our commit log guidelines.
+Before your commit/push, make sure follow our commit log guidelines.
 
 The outline is as below:
 ```bash
@@ -36,8 +36,8 @@ The outline is as below:
   - **skip**: For commits made by after the 1st commit. Usually for applying code review changes.
 
 - #### Body
-A short descriptive message which commit consists.
-At the end preferably(or should) contain related github issue number referencing by `Ref #ISSUE-NO`.
+A short descriptive message part of the commit.
+At the end, it should preferably contain related github issue number referenced by `Ref #ISSUE-NO`.
 
 ex) When the commit is about issue number 20, then
 ```bash
@@ -64,16 +64,16 @@ skip: Applied the review
 Steps to submit your pull request:
 
 1. Fork `billboard.js` on your repository
-2. Create new branch from your billboard.js master branch (and be sure always to be up-to-date)
+2. Create a new branch from your billboard.js master branch (and be sure to be always up-to-date)
 3. Do your work
 4. Create test code for your work (when is possible)
 5. Run `npm run lint` for linting and code style check. (update until without any error or warnings)
-6. Run test code by `npm test` or `npm test:chrome` to run the test on chrome browser.
-   Make sure tests are all passed at least in latest version of Chrome(mobile/desktop).
-7. Write commit log following convention and push to your repository branch.
+6. Run test code by `npm test` or `npm test:chrome` for chrome browser.
+   Make sure all tests pass at least on the latest version of Chrome(mobile/desktop).
+7. Write a commit log following convention and push to your repository branch.
 8. Create a new PR from your branch to billboard.js.
 9. Wait for reviews.
-   When your contribution is well enough to be accepted, then will be merged to our branch.
+   When your contribution is well enough to be accepted, then it will be merged to our branch.
 10. All done!