Update README.md to add in 'publish message to channel including metadata'

[cameron-michie]

Add in 'publish message to channel including metadata', i.e. in the extras headers json, which I believe has to be done through calling the Message constructor.

[owenpearson]

Looks good, thanks for adding this! a couple of minor suggestions though:

[sacOO7]

Can you add integration test for this?
You can add it below

ably-python/test/ably/realtime/realtimechannel_test.py

Line 53 in b92c770

async def test_subscribe(self):

[owenpearson]

@sacOO7 we already have tests for publishing with extras

ably-python/test/ably/rest/restchannelpublish_test.py

Lines 354 to 371 in b92c770

	# TM2i, RSL6a2, RSL1h
	async def test_publish_extras(self):
	channel = self.ably.channels[
	self.get_channel_name('canpublish:extras_channel')]
	extras = {
	'push': {
	'notification': {"title": "Testing"},
	}
	}
	message = Message(name='test-name', data='test-data', extras=extras)
	await channel.publish(message)
	# Get the history for this channel
	history = await channel.history()
	message = history.items[0]
	assert message.name == 'test-name'
	assert message.data == 'test-data'
	assert message.extras == extras

[sacOO7]

Thanks for pointing out @owenpearson !

[cameron-michie]

Thanks very much guys, appreciate this. I have committed those changes Owen, and hope I've done it correctly!

[owenpearson]

LGTM, thanks!