add README

README.md

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+# README
+
+## Workflow
+
+1. `01-prepare.sh`
+   1. Install R interpreter
+   2. Add required repos to resolve syslibs
+   3. Install pak
+   4. Install awscli for remote s3 sync
+2. `02-build-binaries.sh`
+   1. For every package, do
+      1. Clone from GitHub CRAN mirror
+      2. Install all system dependencies via `pak::local_install_dev_deps()`
+      3. Checkout each available tag
+      4. Build source package
+      5. Build binary package
+      6. Create Archive/ directory
+      7. Move all versions besides the most recent one to Archive/ (for source and binary)
+      8. Create `PACKAGES` metadata file using `cranlike` package
+3. `03-upload.sh`
+   1. Sync all binaries and sources to S3 bucket
+4. `-4-create-archive-rds.sh`
+   1. Create `Meta/archive.rds` file for each package so that `remotes::install_version()` can install archived versions
+   2. Upload `Meta/archive.rds` to S3 bucket
+
+## Support for archived versions
+
+### `remotes::install_version()`
+
+`remotes::install_version()` is searching for a `Meta/archive.rds` file in the `/src/contrib` directory.
+This file must be a list of dataframes containing the information about the archived versions of the package.
+
+Example:
+
+```r
+
+con <- gzcon(url(sprintf("%s/src/contrib/Meta/archive.rds",
+                           c("CRAN" = "https://cloud.r-project.org")), "rb"))
+foo = readRDS(con)
+foo[[1]]
+```
+
+### `pak::pak(package@version)`
+
+`pak` searches for `Archive/<package>` and is able to install all versions existing there.
+Ensure to use a clean cache if other repos have been used before, i.e. call `pak::meta_clean(force = TRUE)` if in doubt or when testing.
+
+## Lessons learned
+
+- Newer R versions have issues compiling C code from older package Versions. Using an older R version to build the binaries is recommended.