Documentation seems to be missing `f` dependency in installation instructions

[Alexander E. Zarebski]

copilot.el/readme.md

Line 151 in 0e24423

 Please make sure you have these dependencies installed (available in ELPA/MELPA): 

The package f is not listed as something you need to have installed but this line suggests it might be needed. Please excuse the noise if this is not the case, and thanks for writing such a helpful package!

[Emil van der Westhuizen]

@aezarebski you're right and this should be added to the readme.