update README for ubuntu 16.04 LTS + custom boost installation details

Summary:
the README didn't have explicit instructions for ubuntu 16.04 LTS...  now it does.

i also added a section for how to handle building folly w/a custom boost installation.
Closes https://github.com/facebook/folly/pull/664

Reviewed By: eduardo-elizondo

Differential Revision: D5694143

Pulled By: yfeldblum

fbshipit-source-id: e2786d045c3459cac49c8b052d90ca24d10f7922

README.md

@@ -83,6 +83,19 @@ folly/test subdirectory as `gtest`:
      rm -f release-1.8.0.tar.gz && \
      mv googletest-release-1.8.0 gtest)
 
+#### Linking non-default boost libraries
+
+If you have boost installed in a non-default location, you need to be sure that
+the linker and configure scripts know where to find boost.  This means making
+sure that the `LIBRARY_PATH` environment variable contains `<BOOST_ROOT>/lib`,
+as well as including the path explicitly when running
+`./configure`:
+
+```
+export LIBRARY_PATH=$BOOST_ROOT/lib:$LIBRARY_PATH
+./configure --with-boost=$BOOST_ROOT/lib
+```
+
 #### Ubuntu 12.04
 
 This release is old, requiring many upgrades. However, since Travis CI runs
@@ -137,7 +150,7 @@ sudo apt-get install \
 
 The above packages are sufficient for Ubuntu 13.10 and Ubuntu 14.04.
 
-In the folly directory, run
+In the folly directory, run:
 ```
   autoreconf -ivf
   ./configure
@@ -146,6 +159,10 @@ In the folly directory, run
   sudo make install
 ```
 
+#### Ubuntu 16.04 LTS
+The packages listed above for 13.10 and 14.04 are sufficient for installation,
+and the build commands remain the same.
+
 #### OS X (Homebrew)
 
 folly is available as a Formula and releases may be built via `brew install folly`.