Readme improvements

[adorton-adobe]

The readme could use some love. Here are a few quick ideas. I'll add more if I think of any.

• Make the intro paragraph more concise
• Link to the UST installer tools
• Move build instructions to user manual appendix (and link to it from readme)

[adorton-adobe]

The Windows build instructions could use some improvement.

• Cygwin isn't necessary or recommended to build the Sync Tool in Windows. make can easily be installed with Chocolatey (choco install make). GNU Make for Windows also works just fine.
• Git should be installed from Git for Windows - https://git-scm.com/download/win
• I've never had to install any C++ libraries to get a Python 3 virtual environment working
• We now ship the required win64 builds for python-ldap in the external directory. This requirement should go away once we switch to ldap3 (see Switch to ldap3 #436).

The upcoming Appveyor build scripts will provide a template for a sane set of build instructions that will work.

[adorton-adobe]

Updates are complete