Update build instructions in CONTRIBUTING.md

This is a temporary, not pretty fix, resulting from the recent port to
jbuilder.

[skip ci]

doc/CONTRIBUTING.md

@@ -88,6 +88,14 @@ git clone https://github.com/your-user-name/lwt.git
 cd lwt/
 ```
 
+If you want to use code coverage, you have to do this (for now):
+
+```
+git checkout -b working-coverage origin/working-coverage
+```
+
+This is a temporary result of the recent port to jbuilder.
+
 We'll use Lwt's own [`opam`][opam-depends] file to install Lwt's dependencies
 automatically. Before doing that, you may want to switch to a special OPAM
 switch just for Lwt:
@@ -99,6 +107,12 @@ opam pin add --no-action lwt .
 opam install --deps-only lwt
 ```
 
+Due to a current bug in the build system, you should also install `react`:
+
+```
+opam install react
+```
+
 [opam-depends]: https://github.com/ocsigen/lwt/blob/8bff603ae6d976e69698fa08e8ce08fe9615489d/opam/opam#L35-L44
 
 On most systems, you should also install libev:
@@ -120,6 +134,12 @@ git checkout -b my-awesome-change
 
 To build Lwt and run its unit tests, first do:
 
+```
+make default-config
+```
+
+or, if working from `origin/working-coverage`:
+
 ```
 ocaml setup.ml -configure --enable-tests
 ```
@@ -168,6 +188,9 @@ Go to the GitHub web interface for your Lwt fork
 button. Follow the instructions, write a nice description, and open the pull
 request.
 
+If you worked from the `working-coverage` branch, select that as the base branch
+instead of `master`!
+
 This will trigger automatic building and testing of your change on many versions
 of OCaml, and several operating systems, in [Travis][travis-ci] and
 [AppVeyor][appveyor-ci]. You can even a submit a preliminary PR just to trigger