Add a basic STATUS document documenting the capabilities supported by dulwich.

STATUS

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+Server capabilities
+===================
+
+The Dulwich server supports the following capabilities:
+
+ * include-tag
+ * thin-pack
+ * multi_ack_detailed
+ * multi_ack
+ * side-band-64k
+ * ofs-delta
+ * no-progress
+ * report-status
+ * delete-refs
+
+Known capabilities that are not supported:
+ * shallow (http://pad.lv/909524)
+
+Client capabilities
+===================
+
+The Dulwich client supports the following capabilities:
+
+ * thin-pack
+ * multi_ack_detailed
+ * multi_ack
+ * side-band-64k
+ * ofs-delta
+ * report-status
+
+Known capabilities that are not supported:
+
+ * shallow
+ * delete-refs
+ * no-progress
+ * include-tag