Plugin docs

Author: nonelse

doc/plugin/README.md

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+# Plugins
+
+The plugin mechanism allows to add optional functionality to the Adjust SDK without changing the current implementation.
+Just add one of the plugins from the `plugin` folder to `Adjust/src/com/adjust/sdk/plugin` 
+and it's functionality will be automatically added.

doc/plugin/Vulcun.md

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+# Vulcun
+
+To add the `Vulcun` plugin, first move the file `plugin/Vulcun.java` to the folder `Adjust/src/com/adjust/sdk/plugin/`.
+
+The `Vulcun` plugin allows to collect the sha-1 of the device's primary email. To access this information, you need to add the following permission to the `AndroidManifest.xml` of your app:
+
+````
+<uses-permission android:name="android.permission.GET_ACCOUNTS" />
+````
+Optionally, you can add a string to be concatenated with the email before is hashed with the `sha-1` algorithm. Add the string in a `meta-data` tag inside the `application` tag, as follows:
+
+```
+<meta-data android:name="AdjustVulcunSalt" android:value="vulcun salt example" />
+```