Created Search operators (markdown)

Search-operators.md

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+Invidious supports search filters provided by YouTube.
+
+Support for more user-friendly search is planned, see [#179](https://github.com/omarroth/invidious/issues/179).
+
+Search filters are currently implemented as `key:value` operators, similar to [DuckDuckGo](https://help.duckduckgo.com/duckduckgo-help-pages/results/syntax/) and other search engines. Operators can be added to the search query to filter results, for example: [`type:playlist sort:views music`](https://invidio.us/search?q=type%3Aplaylist+sort%3Adate+music).
+
+Supported operators:
+
+- `sort:`
+  - `relevance` (default)
+  - `rating`
+  - `upload_date`, `date`
+  - `view_count`, `views`
+- `date:`
+  - `hour`
+  - `today`
+  - `week`
+  - `month`
+  - `year`
+- `type:`
+  - `all` (default)
+  - `video`
+  - `channel`
+  - `playlist`
+  - `movie`
+  - `show`
+- `duration:`
+  - `short`
+  - `long`
+- `features:` Multiple can be specified, for example `features:live,4k,subtitles`
+  - `hd`
+  - `subtitles`
+  - `creative_commons`,`cc`
+  - `live`, `livestream`
+  - `purchased`
+  - `4k`
+  - `360`
+  - `location`
+  - `hdr`
+- `channel:`, `user:`
+  - `UCxxxxxxxxxxxxxxxxxxxxxx`
+  - `author` Can be ambiguous, so using `UCID` is recommended
+- `subscriptions:` If logged in, search only for videos from subscribed channels
+  - `true`
+  - `false`
+
+When using `subscriptions:true` or `channel:UCxxx` other filters are not applied.