Added tips and tricks document to the docs folder

docs/tips-and-tricks.md

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+# Tips and Tricks
+This page contains a few 'tips and tricks' for getting stac-fastapi working in various situations.
+
+## Get stac-fastapi working with CORS
+CORS (Cross-Origin Resource Sharing) support may be required to use stac-fastapi in certain situations. For example, if you are running
+[stac-browser](https://github.com/radiantearth/stac-browser) to browse the STAC catalog created by stac-fastapi, then you will need to enable CORS support.
+
+To do this, edit `stac_fastapi/sqlalchemy/stac_fastapi/sqlalchemy/app.py` (or the equivalent in the `pgstac` folder) and add the following import:
+
+```
+from fastapi.middleware.cors import CORSMiddleware
+```
+
+and then edit the `api = StacApi(...` call to add the following parameter:
+
+```
+middlewares=[lambda app: CORSMiddleware(app, allow_origins=["*"])]
+```
+
+If needed, you can edit the `allow_origins` parameter to only allow CORS requests from specific origins.
+
+## Enable the Context extension
+The Context STAC extension provides information on the number of items matched and returned from a STAC search. This is required by various other STAC-related tools, such as the pystac command-line client. To enable the extension, edit `stac_fastapi/sqlalchemy/stac_fastapi/sqlalchemy/app.py` (or the equivalent in the `pgstac` folder) and add the following import:
+
+```
+from stac_fastapi.extensions.core.context import ContextExtension
+```
+
+and then edit the `api = StacApi(...` call to add `ContextExtension()` to the list given as the `extensions` parameter.