ISTO-30 Subontology loading methods - add reason.

docs/loading-snomed.md

@@ -58,3 +58,27 @@ Simply replace the `--import` argument above with `--import-full` or the `type`
 ## Stopping After Loading
 
 To shutdown Snowstorm after loading data automatically include the `--exit` flag.
+
+## Loading a Subontology Snapshot
+A subontology of SNOMED CT can be generated using the [SNOMED Subontology Extraction project](https://github.com/IHTSDO/snomed-subontology-extraction). This can create an RF2
+snapshot archive to be loaded into a terminology server.
+
+Subontology packages are like edition packages in that they contain (some of) the International Edition. There are two alternative options for loading these into
+Snowstorm.
+
+### Option A - Host a subontology and no other SNOMED CT content
+This approach is suited to those wanting to try SNOMED CT via the IPS ontology. No SNOMED CT licence is needed.
+
+Steps:
+- Simply start with an empty Snowstorm instance and load the snapshot onto the MAIN branch.
+
+### Option B - Host a subontology alongside other content
+This approach is suited to those who already have a Snowstorm instance with content loaded.
+A subontolgy may be added for a number of reasons including hosting a browser for a clinical reference or research group.
+
+Steps:
+- Apply the admin technical-fix `CREATE_EMPTY_2000_VERSION` to create a blank version of the root code system, with effective time 20000101.
+- Create a new code system for the subontology using 20000101 as the `dependantVersionEffectiveTime`.
+    - For example: code system short name `SNOMEDCT-IPS`, branch path `MAIN/SNOMEDCT-IPS`.
+- Load the snapshot into the branch of the new code system.
+- No content will be inherited from the root code system because the subontology code system is dependant on a version of the root code system that has no content.