Clarify the scientific name null values and relation to common names

README.md

@@ -131,7 +131,7 @@ This dataset is a collection of images with associated text. The text matched to
   - `inat21_cls_name`: `<Kingdom>_<Phylum>_<Class>_<Order>_<Family>_<Genus>_<species>` as labeled by iNaturalist.
   - `inat21_cls_num`: Number assigned by iNat21 to the given species (unique identifier for that species within iNat21 dataset).
   The remaining terms describe the _Linnaean taxonomy_ of the subject of the image; they are sourced as described in [Annotation Process, below](#annotation-process).
-  - `scientific_name`: scientific name of the subject of the image. This is the Latin term from the most specific available rank, e.g., `<Genus> <species>`. If only down to "family" is available, it uses that term for scientific name.) There are 394,139 unique scientific names. This column is non-null for all entries; unresolved scientific names are denoted `''`. 
+  - `scientific_name`: scientific name of the subject of the image. This is the Latin term from the most specific available rank, e.g., `<Genus> <species>`. If only down to "family" is available, it uses that term for scientific name.) There are 394,139 unique scientific names. Unresolved scientific names are left null; these entries have common names only. 
   - `kingdom`: kingdom to which the subject of the image belongs (`Animalia`, `Plantae`, `Fungi`, `Chromista`, `Protozoa`, `Bacteria`, `Viridiplantae`, `Protista`, `Orthornavirae`, `Bamfordvirae`, `Archaea`, or `Shotokuvirae`). Note: this large number of kingdoms (12) are considered in recognition of the fact that there is not agreement on merging them. Unresolved kingdoms are denoted `' '`.
   - `phylum`: phylum to which the subject of the image belongs. There are 127 unique phyla.
   - `class`: class to which the subject of the image belongs. There are 368 unique classes; due to the change in taxonomic reference for more faithful representation, as discussed in [Curation Rationale](#curation-rationale), some classes (e.g., _Reptilia_) are no longer included.