Update page 'Filtering'

Filtering.md

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+Filters can be used in several places, either in a Filter Stage to directly filter items in feeds, or as conditions for a Conditional Stage to control whether or not another pipeline stage is applied to an item.
+
+## Filters
+A filter is a collection of [Rules](#rules) along with information about how they should collectively apply to an item.
+
+### `mode`
+Either `accept` or `reject`. Accepting rules will allow any items that match the filter whereas rejecting rules will block any items that match.
+
+### `score`
+Items with a score greater than or equal to the filter score are considered to match the filter.
+
+### `rules`
+The array of rules, each of which will be checked against the RSS item.
+
+## Rules
+Each rule has several pieces of data:
+
+### `mode`
+Either `contains_string`, which checks if the property contains the string in the `param` field or `matches_regex`, which checks if the property matches the regular expression in the `param` field.
+
+### `property`
+Which property of the RSS item the rule will check. One of `url`, `title`, or `author`.
+
+### `param`
+The parameter of the rule. Usage depends on the rule's `mode`.