document config.json (#88)

publish/troubleshoot-build.qmd

@@ -11,12 +11,26 @@ title: "Trouble shoot package builds"
 
 ## Can the package build process be customized with custom options/tools/variables?
 
-No, customization is not possible, except for addressing third-party system requirements (covered in the next section).  
+No, customization is overall not possible, except for addressing third-party system requirements (covered in the next two sections).  
 Think of R-universe as your own mini-CRAN. 
 The build environment is very similar to CRAN's, so if a package builds successfully on CRAN, it is likely to work on R-universe.  
 
 This restriction ensures that packages deployed on R-universe are reproducible, work seamlessly on user machines, and do not rely on custom settings or tools.  
 
+## Configure some options with `config.json`
+
+You can create a `config.json` file alongside your `packages.json` file to set some global options.
+
+Currently the only option is `cran_version` which has to be a `yyyy-mm-dd` string and will use dependencies from a certain p3m snapshot date, for example: https://github.com/r-multiverse/staging/blob/main/config.json 
+
+```json
+{
+  "cran_version": "2025-01-15"
+}
+```
+
+This will set `options(repos = c(CRAN = "https://p3m.dev/cran/2025-01-15"))`
+
 ## What if the Git repository  is not a ready-to-build source package?
 
 For repositories requiring additional preparation before building, you can add a [script](https://github.com/r-universe-org/build-source/blob/5830b8aa92d5524b3af6d1b617e605d1a2558543/entrypoint.sh#L50-L57) at the root of the package: