Create CONTRIBUTING.md (#1686)

CONTRIBUTING.md

+# [VuFind Developer Manual](https://vufind.org/wiki/development)
+
+## Prerequisites
+
+ - [Community Code of Conduct](https://github.com/vufind-org/vufind/blob/dev/CODE_OF_CONDUCT.md)
+ - [Coding Standards](https://vufind.org/wiki/development:coding_standards) - How to conform to VuFind's style.
+ - [Recommended Tools](https://vufind.org/wiki/development:recommended_tools) - Suggestions for successful VuFind development.
+ - [Making Pull Requests](https://vufind.org/wiki/development:making_pull_requests)
+ 
+## About VuFind
+
+ - [VuFind Developer Manual](https://vufind.org/wiki/development) - A hub of information on VuFind's design and dependencies.
+ - [Videos](https://vufind.org/wiki/videos) - Tutorials and other videos about VuFind.
+ - [Profiling](https://vufind.org/wiki/development:profiling) - Information on profiling VuFind's code to improve performance.
+ - [Testing](https://vufind.org/wiki/development:testing) - Notes on testing VuFind.
+ - [Troubleshooting](https://vufind.org/wiki/development:troubleshooting) - Notes on troubleshooting and debugging VuFind.
+ - [APIs](https://vufind.org/wiki/development:apis) - APIs that VuFind provides for searching and other functionality.
+ 
+## VuFind Planning
+ - [Community Call](https://vufind.org/wiki/community_call) - A regular meeting (free, online and open to all) for discussion, planning and information sharing.
+ - [GitHub pull requests](https://github.com/vufind-org/vufind/pulls) - A place to view and discuss work in progress on new features.
+ - [JIRA](http://vufind.org/jira) - VuFind's issue tracker.
+ - [Slack](http://vufind.slack.com/) - Email crhallberg@gmail.com for an invitation to the Slack if you cannot sign up directly.