Add contribution templates

CONTRIBUTING.md

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+Thanks for helping out! 😇
+
+### Raising an issue
+
+* Make sure the issue hasn't been raised yet
+* Include **screenshots** and animated GIFs in your issue whenever possible
+* Tag your issue accordingly: bug, enhancement, or question
+
+### Submitting a Pull Request
+
+* Include **screenshots** and animated GIFs in your pull request whenever possible
+* Use the present tense ("Add feature" not "Added feature")
+* Use the imperative mood ("Move cursor to..." not "Moves cursor to...")
+* Limit the first line to 72 characters or less
+* Reference issues and pull requests liberally
+
+### Bulma Sass styleguide
+
+* **No semi-colons** `;` or **curly braces** `{` `}`
+* **No camelCase**
+* Use only **classes**
+* Order the CSS properties **alphabetically**
+* Order the CSS rule by
+  * direct styles
+  * nested tags
+  * pseudo-classes
+  * color modifiers
+  * size modifers
+  * modifiers
+  * responsive styles
+* Add appropriate one-line comments for each of these sections within a CSS rule
+* **No trailing space**
+* End files with a **newline**
+
+```sass
+.element
+  @extend .something
+  +mixin
+  property: value
+  span
+    // ...
+  div
+    // ...
+  .child
+    // ...
+  &:pseudo-class
+    // ...
+  // Colors
+  @each $name, $pair in $colors
+    // Loop
+  // Sizes
+  &.is-small
+    // ...
+  &.is-medium
+    // ...
+  &.is-large
+    // ...
+  // Modifiers (ordered alphabetically)
+  &.has-icons
+    // ...
+  &.is-active
+    // ...
+  &.is-fullwidth
+    // ...
+  // Responsiveness
+  +mobile
+    // ...
+  +tablet
+    // ...
+  +desktop
+    // ...
+```