doc: Improve CONTRIBUTING.md by adding some guidance into the codebas…

…e for people who want to contribute to the code

CONTRIBUTING.md

@@ -1,16 +1,95 @@
-Bug reports and feature requests for **the opam tool** should be reported on:
+Contributions are most welcome in any form.
 
+If you have bug reports and feature requests for the opam tool, please report them at:
 * http://github.com/ocaml/opam/issues
   (please include the output of `opam config report` whenever possible)
 
-**Packaging issues** or requests for a new package should be reported on:
-
+For packaging issues (e.g. packages failed to build) please report them at
 * http://github.com/ocaml/opam-repository/issues
 
-**General queries** can be addressed at:
+## Working on the opam codebase
+
+### Dependencies
+
+To get the dependencies required to build and test opam there are several ways depending on what you have currently.
+
+If you do not have OCaml already installed, you can simply call:
+```
+make cold
+```
+
+If you have OCaml but do not have opam already installed, you can simply call:
+```
+./configure --with-vendored-deps
+```
+
+If you have opam installed already, you can call:
+```
+opam install --deps-only -t .
+```
+
+### Building
+
+As long as you have the dependencies, compiling opam is as simple as:
+```
+./configure
+make
+```
+
+### Running the tests
+
+Once built, you can run the testsuite using:
+```
+make test
+```
+
+If you want to run one test in particular:
+```
+make reftest-<name of the test>
+```
+or
+```
+make reftest-<name of the test> DUNE_ARGS="--auto-promote"
+```
+You can also run the test with `make reftest-<name of the test>, see the diff,
+and call afterwards `dune promote` to promote the new output.
+
+If you just want to run the quickest tests, you can run:
+```
+make quick-reftests
+```
+
+### Adding a new test
+
+If you want to add a new test, create a new `.test` file in `tests/reftests` and run:
+```
+make reftest-gen
+```
+
+### Layout
+
+The source code of opam is located in `src`. In this directory you will find subdirectories.
+Each subdirectories have different puposes:
+* `core`: is where all the lowest level common code used everywhere else is (opam stdlib, IOs, retrocompatibility with older versions of OCaml, code for version handling, …)
+* `format`: depends on `opam-core` and is entirely dedicated to parsing opam files (higher level, but still using `opam-file-format`) and other internal config files
+* `repository`: depends on `opam-format` and gathers code handling everything related to how to download and store repositories. The same code path is also reused when downloading a package for example.
+* `solver`: depends on `opam-format` and gathers everything related to the various options for constraint solving that opam can use (custom search, dose, mccs, z3, 0install, …)
+* `state`: depends on `opam-format` and gathers code dealing with the diverse states of opam (environments, depexts, internal state files handling, pinning, …)
+* `client`: depends on all the above and is where the entry point for the opam binary and all the code handling all the opam subcommands is.
+
+See https://opam.ocaml.org/doc/api/ for more detail.
+
+The tests are all located in the `tests` directory.
+From there:
+* `bench` deals with benchmarks (used by ocurrent-bench)
+* `reftests` is where the majority of the tests are. It consists of a series of cram-like tests (system testing) using a custom syntax parsed and driven by [`tests/reftests/run.ml`](https://github.com/ocaml/opam/blob/master/tests/reftests/run.ml#L12). See below for more details.
+
+### Reftests inner-workings
+
+Each `*.test` file in `tests/reftests` starts with either `N0REP0` (empty repository, note the zeros) or a valid hash coming from opam-repository that the test starts with.
+
+After that, each line starting with `### ` will be parsed as either a command or a custom action as defined by `tests/reftests/run.ml`. Everything else is the output of the command or the input of the custom commands, with the exception of `# Return code <n> #` when the command returns a non-zero exit code.
 
-* http://lists.ocaml.org/listinfo/platform
-  (for the both the tool & packages)
+### Releasing
 
-* http://lists.ocaml.org/listinfo/opam-devel
-  (for the tool and its evolution)
+See release/readme.md