README: other resources section (#7)

Reference recent Crossref dumps discussed in #5.
Closes #5

Refs CrossRef/rest-api-doc#271

README.md

@@ -46,7 +46,7 @@ mongoexport \
 
 See [`data/mongo-export`](data/mongo-export) for more information on `crossref-works.json.xz`.
 Note that creating this file from the Crossref API takes several weeks.
-Users are encouraged to use the cached version available on [figshare](https://doi.org/10.6084/m9.figshare.4816720).
+Users are encouraged to use the cached version available on [figshare](https://doi.org/10.6084/m9.figshare.4816720) (see also [Other resources](#other-resources) below).
 
 [`1.works-to-dataframe.ipynb`](1.works-to-dataframe.ipynb) is a Jupyter notebook that extracts tabular datasets of works (TSVs), which are tracked using Git LFS:
 
@@ -74,6 +74,20 @@ conda env create --file=environment.yml
 
 Then use `source activate crossref` and `source deactivate` to activate or deactivate the environment. On windows, use `activate crossref` and `deactivate` instead.
 
+## Other resources
+
+Ideally, Crossref would provide a complete database dump, rather than requiring users to go through the inefficient process of API querying all works: see [CrossRef/rest-api-doc#271](https://github.com/CrossRef/rest-api-doc/issues/271).
+Until then, users should checkout the Crossref data currently hosted by this repository, whose query date is 2017-03-21, and its corresponding [figshare](https://doi.org/10.6084/m9.figshare.4816720.v1).
+For users who need more recent data, Bryan Newbold [used this codebase](https://github.com/greenelab/crossref/issues/5) to create a MongoDB dump dated January 2018 (query date of approximately 2018-01-10), which he uploaded to the [Internet Archive](https://archive.org/download/crossref_doi_dump_201801).
+His output file `crossref-works.2018-01-21.json.xz` contains 93,585,242 DOIs and consumes 28.9 GB compared to 87,542,370 DOIs and 7.0 GB for the `crossref-works.json.xz` dated 2017-03-21.
+This increased size is presumably due to the addition of [I4OC](https://i4oc.org/ "Initiative for Open Citations") references to Crossref work records.
+This repository is currently seeking contributions to update the convenient TSV outputs based on the January 2018 database dump.
+
+Daniel Ecer also downloaded the Crossref work metadata in January 2018, using the codebase at [elifesciences/datacapsule-crossref](https://github.com/elifesciences/datacapsule-crossref).
+His database dump is available on [figshare](https://doi.org/10.6084/m9.figshare.5845554.v2 "Crossref Works Dump - January 2018").
+While the multi-part format of this dump is likely less convenient than the dumps produced by this repository, Daniel Ecer's analysis also exports a DOI-to-DOI table of citations/references [available here](https://doi.org/10.6084/m9.figshare.5849916.v1 "Crossref Citation Links - January 2018").
+This citation catalog contains 314,785,303 citations ([summarized here](https://elifesci.org/crossref-data-notebook)) and is thus more comprehensive than the catalog available from [greenelab/opencitations](https://github.com/greenelab/opencitations).
+
 ## Acknowledgements
 
 This work is funded in part by the Gordon and Betty Moore Foundation's Data-Driven Discovery Initiative through Grant [GBMF4552](https://www.moore.org/grant-detail?grantId=GBMF4552) to [**@cgreene**](https://github.com/cgreene "Casey Greene on GitHub").