Improve installation guide [ci skip] (#2424)
Improve installation guide [ci skip] (#2424)

- fix GFM
    - fix indentation of nested lists
    - code block
- Add how to specify install location (close #698)
ptosco/rdkit,bp-kelley/rdkit,greglandrum/rdkit,bp-kelley/rdkit,greglandrum/rdkit,rdkit/rdkit,rdkit/rdkit,greglandrum/rdkit,ptosco/rdkit,ptosco/rdkit,bp-kelley/rdkit,bp-kelley/rdkit,bp-kelley/rdkit,greglandrum/rdkit,bp-kelley/rdkit,rdkit/rdkit,greglandrum/rdkit,bp-kelley/rdkit,rdkit/rdkit,greglandrum/rdkit,bp-kelley/rdkit,ptosco/rdkit,ptosco/rdkit,greglandrum/rdkit,rdkit/rdkit,ptosco/rdkit,rdkit/rdkit,rdkit/rdkit,greglandrum/rdkit,rdkit/rdkit,ptosco/rdkit,ptosco/rdkit