Add basic documentation on configuring/utilising octoDNS processors

docs/processors.md

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+# octoDNS processors
+
+## Available processors
+
+These are listed in the main [`README`](../README.md#processors)
+
+## Configuring processors
+
+Configuring processors is done in the main config file.
+
+### Defining a processor configuration
+
+This is done under the top-level `processors` key in the octoDNS config file (for example `config.yaml`), as a sibling to the `manager` key.
+
+The `processors` key contains YAML objects, where the key is the name of the processor, and the `class` value within that object refers to the processor name.
+
+For example, to define a provider called `custom_meta` using the [`MetaProcessor`](../octodns/processor/meta.py) in order to extend the default `include_meta` behaviour:
+
+```yaml
+manager:
+    include_meta: false # disable default, basic `meta` records
+processors:
+    custom_meta:
+        class: octodns.processor.meta.MetaProcessor
+        record_name: meta
+        include_time: true
+        include_uuid: true
+        include_provider: true
+        include_version: false
+```
+
+**NOTE:** the specific parameters for each processor are only documented within [the code](../octodns/processor/)
+
+### Utilising the processor configuration
+
+#### On **individual** domains
+
+Each domain can utilise the processor independently by adding the name of the defined processor to a `processors` key beneath a `zone`:
+
+```yaml
+zones:
+    "example.com":
+        source:
+            - yaml_config
+        target:
+            - hetzner
+        processors:
+            - custom_meta
+```
+
+#### On **all** domains
+
+To utilise the processor on **all** domains automatically, including new domains added to the `zones` config in future then you can add this to the `processors` key under the `manager` section of the configuration:
+
+```yaml
+manager:
+    processors:
+        - custom_meta
+```