Cache update docs

docs/releasehistory.md

@@ -9,6 +9,9 @@ Releases follow the ``major.minor.micro`` scheme recommended by
 
 <!-- ## Since last release -->
 
+### Documentation Updates
+* [#321] - Documents how to download pre-computed QC data and add it to the local cache, which can avoid the need for local calculcations. by [@jthorton]
+
 ### Bug fixes
 * [#320] - Fixes a formatting issue ([#319]) when printing SMILES to summary table. by [@j-wags]
 

docs/users/bespoke-executor.md

@@ -111,6 +111,28 @@ corresponding environment variable [settings] are used instead.
 [QCEngine]: http://docs.qcarchive.molssi.org/projects/QCEngine/en/stable/
 [settings]: openff.bespokefit.utilities.Settings
 
+(executor_qc_cache)=
+## The QC Cache
+Bespokefit makes extensive use of caching to speed up the parameterization process. 
+The generation of the training data is currently the slowest part of the workflow when running DFT calculations with 
+a high level of theory. To further speed up the process we provide an interface to seed the cache with results from 
+[QCArchive] which contains hundreds of torsiondrives. The `cache` command allows you to select a dataset and translate 
+it into local copies of the records which means your molecule data is not shared with QCArchive as the look up is done locally.
+
+First you should start a Bespoke [executor](executor_using_cli) and specify the location of the working directory which will store the cache
+
+```shell
+openff-bespoke executor launch --directory bespoke
+```
+
+While this is running from another terminal run the cache update using any of the available datasets
+
+```shell
+openff-bespoke cache update --no-launch-redis --qcf-dataset "OpenFF-benchmark-ligand-fragments-v2.0" --qcf-address "https://api.qcarchive.molssi.org:443/"
+```
+
+[QCArchive]: https://qcarchive.molssi.org/
+
 (executor_using_api)=
 ## Using the API