Add instructions for installiing fmt from source (#1263)

Summary:
Dovetailing off of https://github.com/facebook/folly/issues/1262 ; Folly's dependency on `fmt` isn't explicitly mentioned nor are from-source install instructions provided. This PR adds a short section to the README describing how to build and install fmt from source.
Pull Request resolved: https://github.com/facebook/folly/pull/1263

Reviewed By: vitaut

Differential Revision: D18622693

Pulled By: yfeldblum

fbshipit-source-id: e43ab8c5524df9decc79cdd0cb2aea3d88826617

README.md

@@ -135,6 +135,19 @@ sudo apt-get install \
     pkg-config
 ```
 
+Folly relies on [fmt](https://github.com/fmtlib/fmt) which needs to be installed from source.
+The following commands will download, compile, and install fmt.
+
+```
+git clone https://github.com/fmtlib/fmt.git && cd fmt
+
+mkdir _build && cd _build
+cmake ..
+
+make -j$(nproc)
+sudo make install
+```
+
 If advanced debugging functionality is required, use:
 
 ```