My example of using a Google sheet

rbm-schedule.php

<?php

/*
Plugin Name: RBM Schedule
Plugin URI: http://realbigplugins.com
Description: Creates a way for our schedule to be quickly and easily referenced.
Version: 0.1
Author: Kyle Maurer
Author URI: http://realbigmarketing.com
License: GPL2
*/

function rbm_get_data() {
	$url     = 'https://spreadsheets.google.com/feeds/list/14Bmvn9V1d91ecbITe4V2V2M-nTSfvvDDGJF7PgEapzM/od6/public/values?alt=json';
	$request = wp_remote_get( $url );
	$request = json_decode( wp_remote_retrieve_body( $request ) );
	$output = '';
	foreach ( $request->feed->entry as $project ) {
		$title   = get_object_vars( $project->title );
		$duedate = get_object_vars( $project );
		$duedate = get_object_vars( $duedate['gsx$duedate'] );
		$output .= $title['$t'] . ' - ' . $duedate['$t'] . '@@';
	}
	return $output;
}

function rbm_post_to_slack() {
	if ( 'go' == $_REQUEST['slack'] ) {
		$text = json_encode( array(
			'text' => rbm_get_data(),
				'username' => 'Schedule Bot',
				''
			)
		);
		$text = str_replace( '@@', '\n', $text );
		$url = 'https://hooks.slack.com/services/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';
		wp_remote_post( $url, array(
				'method' => 'POST',
				'timeout' => 45,
				'redirection' => 5,
				'httpversion' => '1.0',
				'blocking' => true,
				'headers' => array(),
				'body' => $text,
				'cookies' => array()
			)
		);
	}
}
add_action( 'init', 'rbm_post_to_slack' );