docs: add contributing guide

CONTRIBUTING.md

@@ -0,0 +1,18 @@
+#### What should I do before opening an Pull Request?
+
+Before opening an issue, please ensure that:
+  - Use `prettier` to format and style the code
+  - Use `camelCase` to name variables and follow the style of the code
+
+#### What should I do before opening an issue?
+
+Before opening an issue, please ensure that:
+  - You can reproduce the problem with the latest `ls_emmet` release
+  - Nobody else has opened the same issue recently.
+
+#### What should I include in an issue?
+
+When opening a new issue, please include this:
+  - A clear description of the bug
+  - A minimal config wherein the bug could be repreduced
+

README.md

@@ -70,6 +70,10 @@ And it will be expanded to:
 </table>
 ```
 
+#### Contribuiting
+Bug reports, feature suggestions and especially code contributions are most welcome. You open a GitHub issue or pull
+request. Please read [this document](CONTRIBUTING.md) before opening an issue.
+
 #### Supported Languages
 
 ##### markup: