Update README guidance to depend on git tags when building from source. (#134)

*Issue #, if available:*

*Description of changes:* Provide some guidance on how to check out the
repository for applications where build stability is important. This
guidance aims to make itself superfluous as we improve our CI workflows.


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under the terms of the Apache 2.0 license.

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Signed-off-by: Simon Marty <[email protected]>

Author: simonmarty

README.md

@@ -41,6 +41,10 @@ To build the Secrets Manager Agent binary natively, you need the standard develo
 
 ------
 
+**NOTE:** To ensure a stable experience, use a specific git tag when building from source code. You can find a list of version tags [here](https://github.com/aws/aws-secretsmanager-agent/tags). Tags are in the pattern `/v\d+\.\d+\.\d+/` and follow [SemVer 2.0.0](https://semver.org/spec/v2.0.0.html).
+
+Example: `git clone --branch <git tag> https://github.com/aws/aws-secretsmanager-agent.git`
+
 **NOTE:** Building the agent with the `fips` feature enabled on macOS currently requires the following workaround:
 
 - Create an environment variable called `SDKROOT` which is set to the result of running `xcrun --show-sdk-path`