Move old ROADMAP into README

README.md

@@ -51,5 +51,64 @@ build by running:
     cabal install stackage
     stackage nightly
 
-The code itself
+## Processing
----------------
+
+The following describes at a high level the series of steps for processing
+
+### Nightlies
+
+1. Get list of core packages
+2. Get build constraints from list of maintained packages
+3. Load up package index
+4. Calculate build plan using newest versions of packages
+5. Write out a YAML file with complete build plan
+6. Verify that the build plan can be compiled
+7. Perform the build
+
+### LTS
+
+1. Load up most recent build plan
+2. Convert build plan into constraints for next build
+3. Continue from step (3) above
+
+## Code explanation
+
+We start off with *constraints*. Constraints state things like "package X has a
+given version range," who the maintainer is for a package, the description of
+the system/compiler being used, etc. `BuildConstraints` describes the build as
+a whole, whereas `PackageConstraints` describes the constraints on an
+individual package.
+
+There are two primary ways of getting a `BuildConstraints`.
+`defaultBuildConstraints` inspects the first GHC in the PATH environment variable to
+determine GHC version, core packages, core tools, etc. It then uses the
+`Stackage.Config` module to extract information on additional packages to be
+installed. The secondary approach is in `Stackage2.UpdateBuildPlan`, which will be
+discussed later.
+
+`BuildConstraints` does not specify a build completely. That is given by a
+`BuildPlan`, which is similarly broken down into `BuildPlan` and `PackagePlan`.
+In order to get a `BuildPlan`, we need two pieces of information: the
+`BuildConstraints`, and a package index. The package index (usually downloaded
+from Hackage) is a collection of all of the cabal files available.
+
+By applying a `BuildConstraints` to a package index (via `newBuildPlan`), we
+get a proposed `BuildPlan`. There is no guarantee that this `BuildPlan` is
+valid. To validate it, we use `checkBuildPlan`. A `BuildPlan` is an instance of
+both `ToJSON` and `FromJSON`, and therefore can be serialized to a file for
+later use.
+
+When dealing with LTS Haskell, we want to be able to take a `BuildPlan`, and
+update to a newer `BuildPlan` that keeps all packages at the same major
+version.  `updateBuildConstraints` turns a `BuildPlan` into a new
+`BuildConstraints` with that restriction, and `updateBuildPlan` applies
+`newBuildPlan` to that result. As mentioned previously: this is *not* a
+validated result, and therefore `checkBuildPlan` must be used.
+
+A `BuildPlan` can be acted on. This is done to check that all packages compile
+together, run relevant test suites, test Haddock documentation is correct, and
+produce as artifacts both a self-contained GHC binary package database and a
+set of Haddock documentation. (Not yet implemented.)
+
+A `BuildPlan` may be converted into a bundle to be uploaded to Stackage Server.
+(Not yet implemented.)