Improve docs and variable naming

[renefloor]

:sparkles: What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Docs update

:arrow_heading_down: What is the current behavior?

Docs don't mention the breaking changes. Docs are also not clear about when caches are cleared.

:new: What is the new behavior (if this is a feature change)?

The readme has a part about the breaking changes. The config class has documentation about when a cache becomes 'stale'. The variable name of maxAge is changed to 'stalePeriod' as an old file is not deleted if it is still used.

:boom: Does this PR introduce a breaking change?

Yes, maxAgeObject changed names, but it was already a breaking change in the config file, so it is not really useful to add a deprecated variable.

:bug: Recommendations for testing

:memo: Links to relevant issues/docs

Fixes #139 Fixes #106

:thinking: Checklist before submitting

• [X] All projects build
• [X] Follows style guide lines (code style guide)
• [X] Relevant documentation was updated
• [X] Rebased onto current develop

[codecov[bot]]

Codecov Report

Merging #228 into develop will decrease coverage by 0.26%. The diff coverage is 25.00%.

@@             Coverage Diff             @@
##           develop     #228      +/-   ##
===========================================
- Coverage    75.22%   74.95%   -0.27%     
===========================================
  Files           18       18              
  Lines          565      567       +2     
===========================================
  Hits           425      425              
- Misses         140      142       +2     

Impacted Files	Coverage Δ
flutter_cache_manager/lib/src/cache_manager.dart	84.84% <0.00%> (-2.66%)	:arrow_down:
.../lib/src/cache_managers/default_cache_manager.dart	0.00% <ø> (ø)
...utter_cache_manager/lib/src/config/_config_io.dart	12.50% <ø> (ø)
flutter_cache_manager/lib/src/cache_store.dart	92.39% <100.00%> (ø)

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