Add man page for spf_check.sh and create installer to install script into path and install the man page

install.sh

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+#!/usr/bin/env bash
+
+# Install script into path if not already installed
+[[ -x /usr/local/bin/spf_check.sh ]] || sudo ln -sf $(pwd)/spf_check.sh /usr/local/bin/
+
+# Install man page if not already installed
+[[ -f /usr/share/man/man1/spf_check.sh.1 ]] || sudo cp $(pwd)/spf_check.sh.1 /usr/share/man/man1/

spf_check.sh.1

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+.TH spf_check.sh 1 "September 2023" "Version 1.0" "spf_check.sh Manual"
+.SH NAME
+spf_check.sh - A simple command-line tool
+
+.SH SYNOPSIS
+\fBspf_check.sh\fR [domain.tld] [options]
+
+.SH DESCRIPTION
+A simple command-line tool to count the SPF record lookups and optionally display each record.
+
+.SH OPTIONS
+\fBtrue, yes, 1\fR
+Enable debug mode to see each record.
+
+.SH EXAMPLES
+$ spf_check.sh google.com
+
+Total: 31 (Max lookups: 4)
+
+$ ./spf_check.sh google.com true
+
+_spf.google.com
+  _netblocks.google.com
+    35.190.247.0/24
+    64.233.160.0/19
+    66.102.0.0/20
+    66.249.80.0/20
+    72.14.192.0/18
+    74.125.0.0/16
+    108.177.8.0/21
+    173.194.0.0/16
+    209.85.128.0/17
+    216.58.192.0/19
+    216.239.32.0/19
+  _netblocks2.google.com
+    2001:4860:4000::/36
+    2404:6800:4000::/36
+    2607:f8b0:4000::/36
+    2800:3f0:4000::/36
+    2a00:1450:4000::/36
+    2c0f:fb50:4000::/36
+  _netblocks3.google.com
+    172.217.0.0/19
+    172.217.32.0/20
+    172.217.128.0/19
+    172.217.160.0/20
+    172.217.192.0/19
+    172.253.56.0/21
+    172.253.112.0/20
+    108.177.96.0/19
+    35.191.0.0/16
+    130.211.0.0/22
+
+Total: 31 (Max lookups: 4)
+
+.SH AUTHOR
+Christian Sacks <christian@sacks.org.uk>
+
+.SH BUGS
+No known bugs at the moment.
+
+.SH COPYRIGHT
+Copyright (C) 2023 Christian Sacks.