Add package documentation.

diff --git a/src/main/java/net/pterodactylus/reactor/package-info.java b/src/main/java/net/pterodactylus/reactor/package-info.java
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+/**
+ * Reactor main definitions.
+ * <p>
+ * A Reactor chain consists of three different elements: a
+ * {@link net.pterodactylus.reactor.Query}, a
+ * {@link net.pterodactylus.reactor.Trigger}, and an
+ * {@link net.pterodactylus.reactor.Action}.
+ * <p>
+ * A {@code Query} retrieves the current state of a system; this can simply be
+ * the current state of a local file, or it can be the last tweet of a certain
+ * Twitter account, or it can be anything inbetween, or something completely
+ * different.
+ * <p>
+ * After a {@code Query} retrieved the current
+ * {@link net.pterodactylus.reactor.State} of a system, this state and the
+ * previously retrieved state are handed in to a {@code Trigger}. The trigger
+ * then decides whether the state of the system can be considered a change.
+ * <p>
+ * If a system has been found to trigger, an {@code Action} is executed. It
+ * performs arbitrary actions and can use both the current state and the
+ * previous state to define that action.
+ */
+
+package net.pterodactylus.reactor;
+