Add example of a script to add a key to existing account

[justinbarry]

Description

When I was skimming the docs the call to AuthAccount in the middle of the of provided example really threw me off. An additional example would have made it clear.

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• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[turbolent]

Thank you for adding an example for the more common case 👍

[github-actions]

Cadence Benchstat comparison

This branch with compared with the base branch onflow:master commit 1e976a6
The command for i in {1..N}; do go test ./... -run=XXX -bench=. -shuffle=on; done was used.
Bench tests were run a total of 7 times on each branch.

Results

	old.txt	new.txt
	time/op		delta
ParseInfix-2	9.66µs ± 1%	9.99µs ± 6%	+3.39%	(p=0.015 n=6+7)
RuntimeResourceDictionaryValues-2	14.7ms ± 2%	14.8ms ± 3%	~	(p=0.628 n=6+7)
RuntimeFungibleTokenTransfer-2	1.26ms ±26%	1.25ms ±24%	~	(p=0.710 n=7+7)
ParseDeploy/byte_array-2	21.3ms ± 1%	21.1ms ± 2%	~	(p=0.128 n=7+7)
ParseDeploy/decode_hex-2	1.17ms ± 1%	1.17ms ± 1%	~	(p=0.589 n=6+6)
ParseFungibleToken-2	187µs ± 6%	183µs ± 2%	~	(p=0.101 n=7+6)
ParseArray-2	12.9ms ± 1%	13.1ms ± 1%	~	(p=0.240 n=6+6)
QualifiedIdentifierCreation/One_level-2	2.34ns ± 0%	2.35ns ± 0%	~	(p=0.162 n=6+7)
QualifiedIdentifierCreation/Three_levels-2	140ns ± 2%	140ns ± 2%	~	(p=0.859 n=7+6)
CheckContractInterfaceFungibleTokenConformance-2	133µs ± 1%	132µs ± 2%	~	(p=0.534 n=7+6)
NewInterpreter/new_interpreter-2	1.06µs ± 1%	1.07µs ± 1%	~	(p=0.223 n=6+6)
NewInterpreter/new_sub-interpreter-2	2.11µs ± 0%	2.10µs ± 1%	~	(p=0.068 n=7+7)
InterpretRecursionFib-2	2.42ms ± 2%	2.46ms ± 4%	~	(p=0.181 n=6+7)
ContractInterfaceFungibleToken-2	40.1µs ± 1%	39.5µs ± 2%	−1.46%	(p=0.026 n=7+7)
	alloc/op		delta
RuntimeResourceDictionaryValues-2	4.05MB ± 0%	4.05MB ± 0%	~	(p=0.111 n=6+7)
RuntimeFungibleTokenTransfer-2	278kB ± 0%	278kB ± 0%	~	(p=0.805 n=7+7)
QualifiedIdentifierCreation/One_level-2	0.00B	0.00B	~	(all equal)
QualifiedIdentifierCreation/Three_levels-2	64.0B ± 0%	64.0B ± 0%	~	(all equal)
CheckContractInterfaceFungibleTokenConformance-2	66.2kB ± 0%	66.2kB ± 0%	~	(all equal)
ContractInterfaceFungibleToken-2	26.6kB ± 0%	26.6kB ± 0%	~	(all equal)
NewInterpreter/new_interpreter-2	768B ± 0%	768B ± 0%	~	(all equal)
NewInterpreter/new_sub-interpreter-2	1.24kB ± 0%	1.24kB ± 0%	~	(all equal)
InterpretRecursionFib-2	1.24MB ± 0%	1.24MB ± 0%	~	(p=0.972 n=6+7)
	allocs/op		delta
RuntimeResourceDictionaryValues-2	102k ± 0%	102k ± 0%	~	(p=0.359 n=7+7)
RuntimeFungibleTokenTransfer-2	4.62k ± 0%	4.62k ± 0%	~	(p=0.755 n=7+7)
QualifiedIdentifierCreation/One_level-2	0.00	0.00	~	(all equal)
QualifiedIdentifierCreation/Three_levels-2	2.00 ± 0%	2.00 ± 0%	~	(all equal)
CheckContractInterfaceFungibleTokenConformance-2	1.07k ± 0%	1.07k ± 0%	~	(all equal)
ContractInterfaceFungibleToken-2	458 ± 0%	458 ± 0%	~	(all equal)
NewInterpreter/new_interpreter-2	12.0 ± 0%	12.0 ± 0%	~	(all equal)
NewInterpreter/new_sub-interpreter-2	38.0 ± 0%	38.0 ± 0%	~	(all equal)
InterpretRecursionFib-2	25.0k ± 0%	25.0k ± 0%	~	(all equal)