Split up docs. Closes #122

docs/modifying.md

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+## Modifying the source code
+
+Spothole is Public Domain licenced, so you can grab the source code and start modifying it for your own needs.
+Contributions of code back to the main repository are encouraged, but completely optional.
+
+### Code structure
+
+To navigate your way around the source code, this list may help.
+
+*Python back-end code*
+
+* `/core` - Core classes and scripts
+* `/data` - Data storage classes
+* `/spotproviders` - Classes providing spots by accessing the APIs of other services
+* `/alertproviders` - Classes providing alerts by accessing the APIs of other services
+* `/solarconditionsproviders` - Classes providing solar and propagation by accessing the APIs of other services
+* `/server` - Classes for running Spothole's own web server
+
+*Templates*
+
+* `/templates` - Templates used for constructing Spothole's user-targeted HTML pages
+
+*HTML/JS/CSS front-end code*
+
+* `/static` - Root for static files served by the web server. These are all served from a path starting `/static/`.
+* `/static/apidocs` - Contains the OpenAPI spec (`openapi.yml`)
+* `/static/css` - CSS files used by the web front-end
+* `/static/img` - image files used by the web front-end
+* `/static/js` - JavaScript used by the web front-end
+* `/static/vendor` - Third-party libraries (CSS, JS, fonts and images)
+
+*Miscellaneous*
+
+* `/` - Main script (`spothole.py`), pip `requirements.txt`, config, README, etc.
+* `/docs` - Documentation
+* `/images` - Image sources
+* `/datafiles` - Local data sources (differentiated from the majority of data files which are loaded from URLs and
+  cached in `/cache`)
+* `/cache` - Directory where static-ish data downloaded from the internet is cached to avoid rapid re-requests, and
+  where spot/alert data is cached so that it survives a software restart. Created on first run.
+
+### Extending the server
+
+Spothole is designed to be easily extensible. If you want to write your own spot provider, for example, simply add a
+module to the `spotproviders` package containing your class. (Currently, in order to be loaded correctly, the module (
+file) name should be the same as the class name, but lower case.)
+
+Your class should extend "SpotProvider"; if it operates by polling an HTTP Server on a timer, it can instead extend "
+HTTPSpotProvider" where some of the work is done for you.
+
+The class will need to implement a constructor that takes in the `provider_config` and provides it to the superclass
+constructor, while also taking any other config parameters it needs.
+
+If you're extending the base `SpotProvider` class, you will need to implement `start()` and `stop()` methods that start
+and stop a separate thread which handles the provider's processing needs. The thread should call `submit()` or
+`submit_batch()` when it has one or more spots to report.
+
+If you're extending the `HTTPSpotProvider` class, you will need to provide a URI to query and an interval to the
+superclass constructor. You'll then need to implement the `http_response_to_spots()` method which is called when new
+data is retrieved. Your implementation should then call `submit()` or `submit_batch()` when it has one or more spots to
+report.
+
+When constructing spots, use the comments in the Spot class and the existing implementations as an example. All
+parameters are optional, but you will at least want to provide a `time` (which must be timezone-aware) and a `dx_call`.
+
+Finally, simply add the appropriate config to the `spot_providers` section of `config.yml`, and your provider should be
+instantiated on startup.
+
+The same approach as above is also used for alert providers.