Fill the README.

Explain briefly what Yorkie is about and explicitly list the
required dependencies.

README.md

@@ -1,6 +1,31 @@
 Yorkie - A task-oriented GnuPG frontend
 =======================================
 
+Description
+-----------
+Yorkie is a GTK3-based, task-oriented graphical frontend for
+[GnuPG](https://gnupg.org/). “Task-oriented” means that, when it is
+called, the frontend performs a single task (e.g., encrypting a file),
+then quits. Therefore Yorkie is not a replacement for long-running
+frontends such as [GPA](https://gnupg.org/software/gpa/). The intended
+use for Yorkie is to be called from the context menu of a graphical file
+manager, to perform cryptographic operations on files.
+
+The frontend gets its name from *yorkie* (symbol *yki*), a fruit fly
+gene coding for a transcriptional co-activator that is a part of the
+Hippo signaling pathway.
+
+
+Install
+-------
+Yorkie depends on the following libraries:
+
+- [libgpgme](https://gnupg.org/gpgme.html), to perform all cryptographic
+  operations.
+
+- [GTK3](https://gtk.org/) (and underlying libraries such as GLib), for
+  the graphical user interface.
+
 
 Copying
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