Create CONTRIBUTING.md

A link to this doc will be displayed at the top of the 'new issue' screen. Additions or corrections are welcome.
Corrected internal links.
Polishing zh translation, 'properties' section of Configuration.
Improve the zh-translation, 'settings' part of configuration.
Polishing the java doc.
Improve the zh-doc, 'typeAliases' section of configuration
Improve the zh-doc, 'typeHandlers' section of configuration
Update java-api.xml

remove wrong spaces
Merge pull request #247 from htynkn/patch-1

Update java-api.xml
Merge pull request #246 from wangdx/master

Some zh-docs improving and one javadoc polishing.
Improve zh-doc, 'Handling Enums' section of configuration.
Improve zh-doc, 'objectFactory' and 'plugins' section of configuration.
Update logging.xml

fix a typo
Polishing the en-doc, remove the duplicated word.
Improve zh-doc, 'environments','databaseIdProvider' and 'mappers' sections of configuration.
Merge pull request #250 from htynkn/patch-2

fix a typo in document of logging
Merge pull request #251 from wangdx/master

Improve the rest of configuration doc and one duplicated word in en doc.
Improve zh-doc, beginning of the Mapper.
Improve zh-doc, 'select' of Mapper.
Improve zh-doc, 'insert, update and delete' section of Mapper.