Add note about setting PKG_CONFIG_PATH

README.7

@@ -88,6 +88,17 @@ make all
 sudo make install PREFIX=/usr/local
 .Ed
 .
+.Pp
+If your system installs LibreSSL
+in a non-standard path, set
+.Ev PKG_CONFIG_PATH
+for
+.Nm ./configure .
+For example,
+.Bd -literal -offset indent
+PKG_CONFIG_PATH=/opt/libressl/lib/pkgconfig ./configure
+.Ed
+.
 .Sh FILES
 .Bl -tag -width "complete.c" -compact
 .It Pa chat.h