Add intro link in development

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docs/development/README.md

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 Please get in touch with us using our [develompment forum](https://community.openproject.com/projects/openproject/forums/7) or send us an email to info@openproject.org.
 
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 ## Issue tracking and coordination
 
 We eat our own ice cream so we use OpenProject for roadmap planning and team collaboration. Please have a look at the following pages:
 
 - [Development roadmap](https://community.openproject.com/projects/openproject/work_packages?query_id=1993)
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 - [Wish list](https://community.openproject.com/versions/26)
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 - [Bug backlog](https://community.openproject.com/versions/136)
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 - [Reporting a bug](https://www.openproject.org/development/report-a-bug/)
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 - [Submit a feature idea](https://www.openproject.org/development/submit-feature-idea/)
 
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 ## Development Environment
 
 Take a look at the bottom under Additional resources to see how to setup your development environment.
 
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 ## Branching model and development flow
 
 Please see this separate guide for the [git branching model and core development](git-workflow).
 
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+## Development concepts
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+We prepared a set of documentation concepts for an introduction into various backend and frontend related topics of OpenProject. Please see the [concepts main page](concepts/) for more.
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 ## Translations
 
 If you want to contribute to the localization of OpenProject and its plugins you can do so on the [Crowdin OpenProject page](https://crowdin.com/project/openproject). Once a day we fetch those locales and automatically them to GitHub. Contributing there will ensure your language will be up to date for the next release!