Add PREREQUISITES and REFERENCES sections

README

@@ -1,3 +1,9 @@
+PREREQUISITES
+
+	LibreSSL's `libtls'[1], and pkg-config(1) or pkgconf(1)
+	for automatic configuration.
+	`libbsd' for readpassphrase(3) in gemini-cat(1).
+
 BUILDING
 
 	To build libgeminiclient(3) configured automagically:
@@ -54,3 +60,6 @@ PREPROCESSOR DEFINES
 	``BUFFER_SIZE''
 		The size of the buffer for gemini-cat(1) to use,
 		and the default buffer size for gemini-pipe(1).
+
+REFERENCES
+	1: https://www.libressl.org/