update readme for ubuntu 20.04 package

Author: brettkoonce

README.md

@@ -149,6 +149,8 @@ sudo apt-get install    \
 **Note:** LLDB currently requires at least `swig-1.3.40` but will successfully build
 with version 2 shipped with Ubuntu.
 
+**Note:** For Ubuntu 20.04, use `libpython2-dev` in place of the libpython-dev package above.
+
 Build instructions for Ubuntu 14.04 LTS can be found [here](docs/Ubuntu14.md).
 
 ### Getting Sources for Swift and Related Projects