Add installation instructions
Add installation instructions

Amazing that we went so long without having this, but here it is.

Mainly motivated by me wanting to add docs for Docker as suggested by
@ramiro at
https://github.com/pymssql/pymssql/pull/258#issuecomment-51534255

klothe/pymssql,JimDennis/pymssql,ramiro/pymssql,Aloomaio/pymssql,zerolugithub/pymssql,Aloomaio/pymssql,ramiro/pymssql,JimDennis/pymssql,zerolugithub/pymssql,ramiro/pymssql,zerolugithub/pymssql,pymssql/pymssql,klothe/pymssql,pymssql/pymssql,klothe/pymssql,Aloomaio/pymssql,JimDennis/pymssql