Add links to feature docs

[Joseph Reynolds]

I want to make it easier to find documentation for features of OpenBMC.  
The README [1] file lists the features and links to the docs repo, but 
not all features are documented there, and I'm still new to BMCs, so it 
is hard for me to determine which docs to read.

I propose to add a link from each feature to its canonical 
documentation.  So if you see me asking question in this area, you'll 
know why.

- Joseph

[1]: 
https://github.com/openbmc/openbmc/blob/master/README.md#features-of-openbmc