[README] Align heading marker

By the way, how about using leading "#"s for heading markup? If we use
leading "#"s instead of underline markup, we don't care about how many
underline should be put.

The current style:

    Supported platforms
    -------------------

The suggested style:

    ## Supported platforms
Merge pull request #270 from project-hatohol/align-heading-marker

[README] Align heading marker