Add documentation for extensions

docs/08_extensions.md

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+# NewPipe extensions
+
+## File format
+
+A NewPipe extension is a signed JAR file containing the following files:
+
+### `about.json`
+
+This is a JSON file describing things about the extension. It should have the following fields:
+
+- `name`: This should be the name of the extension. It should be exactly the same as the name returned by `StreamingService$ServiceInfo#getName()`. If the extension replaces a built-in service, its name should be exactly the same as the name of the built-in service.
+- `author`: This should be the name(s) of the author(s) of the extension.
+- `class`: This should be the fully qualified name of the class that extends `StreamingService`. It should be located in a subpackage of `org.schabi.newpipe.extractor.extensions.services`.
+- `major_version`: The major internal version of NewPipe Extractor that this extension is compatible with.
+- `minor_version`: The minor internal version of NewPipe Extractor that this extension is compatible with. It will also work on later minor internal versions of NewPipe Extractor.
+- `replaces` (optional): The ID of the service that this extension replaces.
+
+### `classes.dex`
+
+This is the Dalvik Executable compiled from your Java code.
+
+### `icon.png`
+
+This is a PNG with a resolution of 512x512 containing the logo of the service in white on a transparent background.
+
+## Runtime environment
+
+You could add the following compile-only dependencies to your project:
+
+- `com.github.TeamNewPipe:NewPipeExtractor`
+- `com.github.TeamNewPipe:nanojson`
+- `org.jsoup:jsoup`
+- `com.github.spotbugs:spotbugs-annotations`
+
+For versions, please check the `build.gradle` of the version of NewPipe Extractor you want to target.
+
+For security reasons, only a limited amount of classes could be used.
+
+### Allowed classes
+
+- `dalvik.annotation.*`
+- `javax.annotation.*`
+- `java.lang.*`
+- `java.util.*`
+- `java.text.*`
+- `com.grack.nanojson.*`
+- `org.schabi.newpipe.extractor.*`
+- `java.io.IOException`
+- `java.io.UnsupportedEncodingException`
+- `java.io.InputStream`
+- `java.net.URL`
+- `java.net.MalformedURLException`
+- `java.net.URLEncoder`
+- `java.net.URLDecoder`
+- `java.net.URI`
+- `java.net.URISyntaxException`
+- `org.jsoup.Jsoup`
+- `org.jsoup.nodes.Document`
+- `org.jsoup.nodes.Element`
+- `org.jsoup.parser.Parser`
+- `org.jsoup.select.Elements`
+
+### Disallowed classes
+
+- `java.lang.reflect.*`
+- `org.schabi.newpipe.extractor.services.*`
+- `java.lang.Class` (this unfortunately means you can't define `enum`s)
+- `java.lang.ClassLoader`
+- `org.schabi.newpipe.extractor.ServiceList`