Frequency of letters in a file

Your script should be able to open a file named words.txt located in the same folder, compute the frequency of every ascii lowercase letters in its content, and display them according to this format:

b: 0.02
p: 0.02
a: 0.07
o: 0.05
s: 0.09
d: 0.03
i: 0.07
m: 0.02
c: 0.03

I do not care about the order, only the values. No more precision, no less precision, and I want the trailing zeros, meaning 1/2 should be printed 0.50.