add instructions to install fmt

[coldtomatojuice]

recently I tried to bulid Clipper using 0.4 release clone, and I got the error message as below

`CMake Error at CMake/folly-deps.cmake:214 (find_package): Could not find a package configuration file provided by "fmt" with any of the following names:

fmtConfig.cmake
fmt-config.cmake

Add the installation prefix of "fmt" to CMAKE_PREFIX_PATH or set "fmt_DIR" to a directory containing one of the above files. If "fmt" provides a separate development package or SDK, be sure it has been installed. Call Stack (most recent call first): CMakeLists.txt:113 (include) `

It seems there have been some minor changes in folly therefore it needs to install fmt from source before install folly

check below issue for the details https://github.com/facebook/folly/pull/1263

thanks

[AmplabJenkins]

Can one of the admins verify this patch?

[robberlang]

This helped me, thanks. The git clone line (line 20) needs a backslash at the end of it though.