update contributing and README partially telling an easy way to get arcanist.

CONTRIBUTING.md

@@ -12,6 +12,7 @@ Before starting, you need a few tools.
   - Ubuntu 16.04 LTS and onwards
  - The following packages from your Linux distribution's repository:
   - `git`
+  -  `arcansit`
   - `python3-sphinx`
   - `texlive-full`
  - The following packages from PyPI:

README.md

@@ -9,7 +9,7 @@ The documentation is powered by [Sphinx](http://www.sphinx-doc.org/en/stable/) a
 Run the following command to install the needed dependencies:
 
 ```
-sudo apt install python3-pip python3-sphinx texlive-full && pip3 install sphinx_bootstrap_theme
+sudo apt install python3-pip python3-sphinx texlive-full && pip3 install sphinx_bootstrap_theme arcanist
 ```
 
 You can see the options for building the documentation by running `make help`.