[review] Improvements to installation instructions

[malramsay64]

Hi @hema-ted,

I am working through the installation steps and there are a few points which require clarification in the documentation.

• [x] The documentation notes that legacy versions of ase are required for python 2, there needs to be a version number known to work with python 2.
• [x] The note about python compatibility, and that python is required for installation should probably be at the start of the installation guide since it needs to be set up at the start of the process.
• [x] Include minor version numbers in the python versions supported, for example python 2.7 and python 3.5+

[hema-ted]

Hi @malramsay64,

Thanks a lot for your comments! I have included your suggested improvements in commit 0c48667. All three points were addressed.

[malramsay64]

Fantastic, it is now much clearer