'Building your function' README updates

[bmoffatt]

Issue #, if available:

#340

Description of changes:

Updates to the 'Building your function' instructions.

The diversity of Linux distributions for development environments, and the longer-than-originally-anticipated age of Amazon Linux/Amazon Linux 2, has also made the GLIBC compatibility issue somewhat more common, so added some notes about dealing with CGO.

Since this section was first written, Lambda has added support for container Images, and EC2 Graviton. Tried to call that out where relevant, and in the case of container images, added a reference to the relevant docs.aws.amazon.com

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (501c2f3) 72.93% compared to head (b968a06) 72.93%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #543   +/-   ##
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  Coverage   72.93%   72.93%           
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  Files          26       26           
  Lines        1456     1456           
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  Hits         1062     1062           
  Misses        322      322           
  Partials       72       72           

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