docs: Custom Terraform Provider

Signed-off-by: Dennis Kniep <[email protected]>

README.md

@@ -185,7 +185,42 @@ Build binary:
 make build
 ```
 
-## Regression Tests 
+### Custom Terraform Provider
+
+If you want to build this crossplane provider on top of a forked `terraform-provider-keycloak` follow these instructions:
+
+1. Execute code generation:
+```
+TERRAFORM_PROVIDER_REPO=https://github.com/<owner>/terraform-provider-keycloak \
+TERRAFORM_PROVIDER_VERSION=1.0.0 \
+make generate
+```
+**Hint:** `TERRAFORM_PROVIDER_VERSION` must be a Release. Releases can be found here: `https://github.com/<owner>/terraform-provider-keycloak/releases`. 
+Every ReleaseName should have the prefix "v" (i.e 'v1.0.0'). But if you specify the `TERRAFORM_PROVIDER_VERSION` you need to 
+skip that prefix (i.e. '1.0.0')
+
+2. Use forked repo as go dependency:
+```
+go mod edit -replace="github.com/keycloak/[email protected]=github.com/<owner>/[email protected]"
+go mod tidy
+```
+**Hint:** You can also specify the version as `github.com/<owner>/[email protected]<timestamp>-<commitHash>`
+
+3. Build and publish to custom repo
+
+If you want to build and publish with CI, then add following to [GithubRepo > Settings > Secrets and variables > Actions](https://github.com/denniskniep/provider-keycloak/settings/secrets/actions)
+
+Variables > Repository variables
+* TERRAFORM_PROVIDER_REPO=https://github.com/<owner>/terraform-provider-keycloak
+* TERRAFORM_PROVIDER_VERSION=1.0.0
+* UPBOUND_MARKETPLACE_PUSH_ROBOT_USR=<user>
+* XPKG_REG_ORGS=xpkg.upbound.io/<repo>
+* XPKG_REG_ORGS_NO_PROMOTE=xpkg.upbound.io/<repo>
+
+Secrets > Repository secrets
+* UPBOUND_MARKETPLACE_PUSH_ROBOT_PSW=<password>
+
+## Regression Tests
 TODO: Add regression test docs
 
 ## Report a Bug