Fix typo in introduction
Merge pull request #266 from jvirtanen/fixes/introduction

Fix typo in introduction
favor unless over if with a single negative condition
Merge pull request #274 from hasghari/conditional

favor unless with positive condition over if with negative condition
Add single-quotes to indent example
Merge pull request #276 from thiagogabriel/fix/single-quote-example

Add single-quotes to indent example
naming files
recommendation to have single class/module per file
Merge pull request #277 from deric/master

naming files
Fix a typo in Percent Literals

Found it while reading the section.
Merge pull request #282 from gsamokovarov/patch-1

Fix a typo in Percent Literals
Mention string formatting as an alternative to string interpolation
Don't use Object#to_s on interpolated objects
[Fix #237] Add guideline for unused variables naming
Extend ||= rule
Recommend the use of Hash#values_at
Recommend the use of self-assignment operators
Add a link to the Japanese version.
Merge pull request #285 from 5704k3/addlinkja

Add a link to the Japanese version.
Clarify a statement