Mention that exercise files need to be added to coqproject to be compiled by make

It took me a while of digging through the makefile and the coq makefile to figure out how to get make to check my proofs for me (as much as I like vscoq, I want to ensure that the base coq executable also accepts my proofs). Turns out I needed to uncomment a line in _CoqProject.

I suggest to write down that information in the README for posterity.

I can't open merge requests on this website, so here's a diff of the changes I'm suggesting:

diff --git a/README.md b/README.md
index 0233f9e..7464344 100644
--- a/README.md
+++ b/README.md
@@ -50,5 +50,7 @@ make -j4
 **IMPORTANT:** You will have to compile to interactively do proofs. You will have to recompile if the dependencies of the file your editing change.
 
+By default, the exercise proofs won't be compiled by the above command.
+You can still work on them interactively, but if you want `make` to check them, you will need to append their path to the `_CoqProject` file.
 
 ## Editing the Coq code
 To edit the Coq code and complete the exercises, you will need an editor. We recommend you use [CoqIDE](https://coq.inria.fr/refman/practical-tools/coqide.html) or [VsCoq](https://github.com/coq-community/vscoq) ([VSCode](https://code.visualstudio.com/)).

- Emilie Burgun