Extend core ideas documentation (#3405)

Co-authored-by: Robert Kaussow <xoxys@rknet.org>

docs/docs/92-development/01-getting-started.md

@@ -1,12 +1,5 @@
 # Getting started
 
-## Core ideas
-
-- A (e.g. pipeline) configuration should never be [turing complete](https://en.wikipedia.org/wiki/Turing_completeness) (We have agents to exec things 🙂).
-- If possible follow the [KISS principle](https://en.wikipedia.org/wiki/KISS_principle).
-- What is used most should be default.
-- Keep different topics separated, so you can write plugins, port new ideas ... more easily, see [Architecture](./05-architecture.md).
-
 You can develop on your local computer by following the [steps below](#preparation-for-local-development) or you can start with a fully prepared online setup using [Gitpod](https://github.com/gitpod-io/gitpod) and [Gitea](https://github.com/go-gitea/gitea).
 
 ## Gitpod

docs/docs/92-development/02-core-ideas.md

@@ -0,0 +1,26 @@
+# Core ideas
+
+- A configuration (e.g. of a pipeline) should never be [turing complete](https://en.wikipedia.org/wiki/Turing_completeness) (We have agents to exec things 🙂).
+- If possible, follow the [KISS principle](https://en.wikipedia.org/wiki/KISS_principle).
+- What is used most often should be default.
+- Keep different topics separated, so you can write plugins, port new ideas ... more easily, see [Architecture](./05-architecture.md).
+
+## Addons and extensions
+
+If you are wondering whether your contribution will be accepted to be merged in the Woodpecker core, or whether it's better to write an
+[addon](../30-administration/75-addons/00-overview.md), [extension](../30-administration/100-external-configuration-api.md) or an
+[external custom backend](../30-administration/22-backends/50-custom-backends.md), please check these points:
+
+- Is your change very specific to your setup and unlikely to be used by anyone else?
+- Does your change violate the [guidelines](#guidelines)?
+
+Both should be false when you open a pull request to get your change into the core repository.
+
+### Guidelines
+
+#### Forges
+
+A new forge must support these features:
+
+- OAuth2
+- Webhooks