Adding import documentation

docs/Adding_classes_from_another_ontology.md

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+# How to add (import) classes to the Cell Ontology (CL) from another ontology
+
+## 1. Follow steps 1 - 5 under the heading [Protege-based declaration.](https://obophenotype.github.io/cell-ontology/odk-workflows/UpdateImports/#protege-based-declaration)
+
+_NB: Even though the instructions state that this workflow is to be avoided, the other solutions in the current documentation are out of date._
+
+
+## 2. Refresh the imports
+
+To refresh the imports, open Docker so it is running in the background. Then open Terminal, navigate to src/ontology directory in the cell-ontology repository and run:
+
+`sh run.sh make imports/merged_import.owl`
+
+Running the above command requires > 8GB RAM and sufficient computational power. If the refresh fails to complete due to hardware limitations, create a new issue in GitHub detailing which class(es) need to be imported and another editor can add it on your behalf.
+
+Once the imports are refreshed, return to Protégé, add the logical axioms that include the newly imported class(es) and create a pull request per standard procedure. 
+
+_Note that the import refresh process seems to be quite laborious/computationally expensive as-is, and a centralised database approach may be an improved longterm solution._