Update README.md

conform to the new attribute namespace.
install mackerel-agent-plugins
fix SYNPOSIS
fix typo.
Merge pull request #19 from ijin/patch-1

Update README.md
Merge pull request #20 from koudaiii/install-mackerel-agent-plugins

add option.Install mackerel agent plugins
Create specific recipe for mackerel-agent-plugins instead of the node attribute
Update README
Merge pull request #21 from mackerelio/mackerel-agent-plugins-recipe

Create specific recipe for mackerel-agent-plugins instead of the node attribute
Add CHANGELOG
Bump version 1.1.0