install.rst: Inform user to restart terminal once they have adjusted their path

After having freshly installed BuildStream on my machine, I found that when I added export PATH=${PATH}:~/.local/bin to the end of my ~/.bashrc file (as the installing instructions suggest), the bst command was still not recognised.

To fix this, you simply restart the terminal or run source ~/.bashrc.

It would be nicer if this was made explicit in the documentation.