docs: explicit encoding/decoding in parsing doc

[aaronlevin]

Give a simple example of encoding and decoding explicitly. There are
many examples of the web of how to do decoding explicitly, but few on
how to do Encoding. This simple example does not expose the whole API,
but will give people a starting point to dig deeper.

PS - it's not clear to me if this is the most idiomatic way (I know there are the tupleN methods, but this seems a little more low-level / basic in a better/education way imo)

[codecov-io]

Codecov Report

Merging #571 into master will decrease coverage by -0.18%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #571      +/-   ##
=========================================
- Coverage   82.57%   82.4%   -0.18%     
=========================================
  Files          72      73       +1     
  Lines        1997    2000       +3     
  Branches      147     147              
=========================================
- Hits         1649    1648       -1     
- Misses        348     352       +4

Impacted Files	Coverage Δ
...main/scala/io/circe/PlatformSpecificPrinting.scala	0% <ø> (-100%)	❌
...r/jvm/src/main/scala/io/circe/parser/package.scala	0% <ø> (-100%)	❌
...d/src/main/scala/io/circe/NonEmptySeqDecoder.scala	90.9% <ø> (-9.1%)	❌
...re/shared/src/main/scala/io/circe/KeyDecoder.scala	43.24% <ø> (-2.71%)	❌
...er/js/src/main/scala/io/circe/parser/package.scala	100% <ø> (ø)
...rc/main/scala/io/circe/numbers/BiggerDecimal.scala	91.35% <ø> (+0.61%)	✅
...main/scala/io/circe/PlatformSpecificPrinting.scala	100% <ø> (+100%)	✅

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[aaronlevin]

I have no idea why this is failing.

[travisbrown]

@aaronlevin The failure is because Scoverage doesn't give stable results and by default Codecov fails the build on any decrease. This is only intermittently an error but it's still a bad experience for contributors and we need to fix it (I think by adding a Codecov config file—I'll either do that now or open an issue).

In any case this looks great to me even if Codecov doesn't like it, so thanks. 😄