Add note in README.md installation through opam

[ghost]

Its trivial but a note in the README would be useful for newcomers (like me) trying to install obus for spotify-cli.

[ghost]

diff --git a/README.md b/README.md
index 8ae3196..98c24ee 100644
--- a/README.md
+++ b/README.md
@@ -24,6 +24,9 @@ For building the development version, you also need to install
 Installation
 ------------

+If you are installing obus as a dependency for another project, 
+then the quickest way to get obus would be, `opam install obus`.
+
 To build and install obus:

     $ ./configure

[ghost]

Indeed, I added a note in the README

[ghost]

Thank you :)