Enable tag citations (#123)

* Enable tag citations

See greenelab/deep-review#2 (comment).

* Reflow text to satisfy codeclimate

CONTRIBUTING.md

@@ -35,18 +35,29 @@ additional feedback.
 
 ## Markdown
 
-The paper will be written using markdown. Markdown files use the `.md` extension.
-Check out the [CommarkMark Help](http://commonmark.org/help/) page for an introduction
-to formatting options provided by markdown. In addition, to standard markdown features,
-we support [markdown tables](https://help.github.com/articles/organizing-information-with-tables/
-"GitHub Help: Organizing information with tables") and a custom citation syntax. Check out
-[Tables Generator](http://www.tablesgenerator.com/markdown_tables) for creating markdown tables.
-
-The custom citation guidelines are as follows: 
-
-1. Always use a DOI for the version of record if available. Cite DOIs like `[@doi:10.15363/thinklab.4]`.
-2. If the version of record doesn't have a DOI but does have a PubMed ID, cite like `[@pmid:26158728]`.
-3. If the article is an _arXiv_ preprint, cite like `[@arxiv:1508.06576]`.
-4. If the article has none of the above, please file an issue.
-
-You cite multiple items at once like `[@doi:10.15363/thinklab.4 @pmid:26158728 @arxiv:1508.06576]`.
+The paper will be written using markdown. Markdown files use the `.md`
+extension. Check out the [CommarkMark Help](http://commonmark.org/help/) page
+for an introduction to formatting options provided by markdown. In addition, to
+standard markdown features, we support [markdown
+tables](https://help.github.com/articles/organizing-information-with-tables/
+"GitHub Help: Organizing information with tables") and a custom citation syntax.
+Check out [Tables Generator](http://www.tablesgenerator.com/markdown_tables) for
+creating markdown tables.
+
+The custom citation guidelines are as follows:
+
+1. Always use a DOI for the version of record if available. Cite DOIs like
+  `[@doi:10.15363/thinklab.4]`.
++ If the version of record doesn't have a DOI but does have a PubMed ID, cite
+  like `[@pmid:26158728]`.
++ If the article is an _arXiv_ preprint, cite like `[@arxiv:1508.06576]`.
++ If the article has none of the above, please file an issue.
+
+You cite multiple items at once like `[@doi:10.15363/thinklab.4 @pmid:26158728
+@arxiv:1508.06576]`.
+
+For improved readability, you can (but are not required to) use a tag. For
+example, you can add a reference to the tag `Zhou2015_deep_sea` using the
+following syntax: `[@tag:Zhou2015_deep_sea]`. If you add references that use
+tags, make sure to add those tags and their corresponding citations to
+[`references/tags.tsv`](references/tags.tsv).