chore(docs): Add note that PRs should be submitted against the develop branch (visjs#2623)

Related to: visjs#2618
Related to: visjs#2620

Author: ypid

CONTRIBUTING.md

@@ -9,6 +9,7 @@ If you have any *general question* on how to use the vis.js library in your own
 If you really want to open a new issue:
 * Please use the [search functionality](//github.com/almende/vis/issues) to make sure that there is not already an issue concerning the same topic.
 * Please make sure to **mention which module** of vis.js (network, timeline, graph3d, ...) your are referring to.
+* Note that development happens on the `develop` git branch. Be sure to submit PRs against this branch.
 * If you think you found a bug please **provide a simple example** (e.g. on [jsbin](jsbin.com)) that demonstrates the problem.
 * If you want to propose a feature-request please **describe what you are looking for in detail**, ideally providing a screenshot, drawing or something similar.
 * **Close the issue later**, when the issue is no longer needed.