Translated '16 Read Preferences' into Spanish. Close #351.
Minor edits for #351.
Merge pull request #376 from puentesarrin/issue-351

Completed translation of "16 Read Preferences" into Spanish. Close #351
Translated 19-Building_a_Sharded_Environment. Resolves #366
Merge branch 'master' of https://github.com/MongoDBPeru/subte