Add missing Makefile to "installing"

[jpcenteno]

Instructions for sequential/installing tell the learner to execute make in order to run the tests, but no makefile was provided.

$> git clone https://github.com/lambdaclass/erlings.git
$> cd ~/erlings/sequential/installing
$> make

---

Got this message:

bash-3.2$ pwd
/Users/lambda/code/erlings/sequential/installing
bash-3.2$ make
make: *** No targets specified and no makefile found.  Stop.

Here is a Makefile to fix that:

joaquin-lambda:installing lambda$ make
rebar3 eunit
===> Verifying dependencies...
===> Compiling installing
===> Performing EUnit tests...
.
Finished in 0.041 seconds
1 tests, 0 failures