contrib: add placeholder manpage for bitcoin binary

Author: fanquake

doc/man/bitcoin.1

@@ -0,0 +1,5 @@
+.TH BITCOIN "1"
+.SH NAME
+bitcoin \- manual page for bitcoin
+
+This is a placeholder file. Please follow the instructions in \fIcontrib/devtools/README.md\fR to generate the manual pages after a release.