Create CONTRIBUTING.md (#372)

CONTRIBUTING.md

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+**All PR are welcome :)**
+
+
+### In a few steps
+1. clone the repo with `git clone --branch dev git@github.com:BlogoText/blogotext.git`
+2. create a new branch `git checkout -b branch-name`
+3. do your work
+4. check your code (see below)
+5. `git push`
+6. Go to your github repository, and create a pull request from your branch to `dev` main repo.
+
+### Few rules
+
+Always work on a dedicated branch:
+
+    git checkout -b branch-name
+
+Before modifying code **and** pushing changes, stay up to date with the `dev` branch:
+
+    // Add blogotext to upstream source
+    git remote add upstream https://github.com/BlogoText/blogotext.git
+
+    // Now, you can refresh your fork from the souce
+    git pull --squash upstream dev
+
+You can found some documentation on [help.github.com / syncing-a-fork](https://help.github.com/articles/syncing-a-fork/)
+
+### Check your code
+
+Travis CI will do it for you, but to prevent errors you can test yourself the code:
+
+    # Download the tool
+    curl -OL https://squizlabs.github.io/PHP_CodeSniffer/phpcs.phar
+
+    # Usage
+    php phpcs.phar --standard=PSR2 -np --tab-width=4 --encoding=utf-8 .
+
+If you are stuck with a **lot of errors**, use this tool which can refactor the code for you:
+
+    # Download the tool
+    curl -OL https://squizlabs.github.io/PHP_CodeSniffer/phpcbf.phar
+
+    # Usage
+    php phpcbf.phar --standard=PSR2 -np --tab-width=4 --encoding=utf-8 .
+
+! phpcbf.phar can not work with window (require diff, path...) you can add ` --no-patch`