Add a detailed README and clean up the configuration file

[cmacknz]

Adds a detailed README file that includes an architecture diagram. Also cleans up the configuration file a little bit and documents all of the fields in the style of the Beats and Agent reference configurations.

It will be easiest to review the rendered README: https://github.com/cmacknz/elastic-agent-shipper/blob/add-readme/README.md

Closes #61

[elasticmachine]

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• Start Time: 2022-08-17T19:27:09.397+0000

• Duration: 10 min 42 sec

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Test	Results
Failed	0
Passed	40
Skipped	0
Total	40

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[cmacknz]

The test failures are all the same. The fix is one one of @rdner's branches:

 === RUN   TestPublish/should_return_validation_errors/no_timestamp
    server_test.go:243: 
        	Error Trace:	server_test.go:243
        	Error:      	Not equal: 
        	            	expected: "timestamp: proto:\u00a0invalid nil Timestamp"
        	            	actual  : "timestamp: proto: invalid nil Timestamp"
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1 +1 @@
        	            	-timestamp: proto: invalid nil Timestamp
        	            	+timestamp: proto: invalid nil Timestamp
        	Test:       	TestPublish/should_return_validation_errors/no_timestamp
        --- FAIL: TestPublish/should_return_validation_errors/no_timestamp (0.00s)