doc: move doc to ci readme

1 files changed, 33 insertions, 6 deletions

diff --git a/ci/README.md b/ci/README.md
index d2ea255b4b..3c5f04c39e 100644
--- a/ci/README.md
+++ b/ci/README.md
@@ -1,12 +1,8 @@
-## ci scripts
+## CI Scripts
 
 This directory contains scripts for each build step in each build stage.
 
-Currently three stages `lint`, `extended_lint` and `test` are defined. Each stage has its own lifecycle, similar to the
-[Travis CI lifecycle](https://docs.travis-ci.com/user/job-lifecycle#the-job-lifecycle). Every script in here is named
-and numbered according to which stage and lifecycle step it belongs to.
-
-### Running a stage locally
+### Running a Stage Locally
 
 Be aware that the tests will be built and run in-place, so please run at your own risk.
 If the repository is not a fresh git clone, you might have to clean files from previous builds or test runs first.
@@ -36,3 +32,34 @@ To run the test stage with a specific configuration,
 ```
 FILE_ENV="./ci/test/00_setup_env_arm.sh" ./ci/test_run_all.sh
 ```
+
+### Configurations
+
+The test files (`FILE_ENV`) are constructed to test a wide range of
+configurations, rather than a single pass/fail. This helps to catch build
+failures and logic errors that present on platforms other than the ones the
+author has tested.
+
+Some builders use the dependency-generator in `./depends`, rather than using
+the system package manager to install build dependencies. This guarantees that
+the tester is using the same versions as the release builds, which also use
+`./depends`.
+
+If no `FILE_ENV` has been specified or values are left out, `00_setup_env.sh`
+is used as the default configuration with fallback values.
+
+It is also possible to force a specific configuration without modifying the
+file. For example,
+
+```
+MAKEJOBS="-j1" FILE_ENV="./ci/test/00_setup_env_arm.sh" ./ci/test_run_all.sh
+```
+
+The files starting with `0n` (`n` greater than 0) are the scripts that are run
+in order.
+
+### Cache
+
+In order to avoid rebuilding all dependencies for each build, the binaries are
+cached and re-used when possible. Changes in the dependency-generator will
+trigger cache-invalidation and rebuilds as necessary.