Remove -I for installing from the source in readme

- Update according to the comment from @mikf
- Follow rst syntax
- remove '--no-cache-dir'

README.rst

@@ -55,7 +55,9 @@ pip_ as well:
 
 .. code:: bash
 
-    python3 -m pip install -U -I --no-deps --no-cache-dir https://github.com/mikf/gallery-dl/archive/master.tar.gz
+    python3 -m pip install -U --force-reinstall --no-deps https://github.com/mikf/gallery-dl/archive/master.tar.gz
+
+Omit :code:`--no-deps` if Requests_ hasn't been installed yet.
 
 Note: Windows users should use :code:`py -3` instead of :code:`python3`.