Address questions regarding suitability for storage and untrusted inputs

[mbr]

Puts some prominent text into the readme.md regarding some use cases
that are likely to be common, along with a few hopefully helpful
pointers to avoid footguns.

Closes #345, closes #216, addresses #240, #266.

[mbr]

Please let me know if the style is fine; as the original readme.md is pleasantly terse. I did opt for putting it into the readme instead of the docs, as people evaluating a format will likely not dive into the docs straight away, but check readmes first.

[codecov-commenter]

Codecov Report

Merging #346 into master will increase coverage by 2.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #346      +/-   ##
==========================================
+ Coverage   61.40%   63.47%   +2.06%     
==========================================
  Files          11       11              
  Lines         824      824              
==========================================
+ Hits          506      523      +17     
+ Misses        318      301      -17     

Impacted Files	Coverage Δ
src/de/mod.rs	78.43% <0.00%> (+0.65%)	⬆️
src/ser/mod.rs	84.40% <0.00%> (+6.42%)	⬆️
src/config/limit.rs	54.54% <0.00%> (+9.09%)	⬆️
src/config/trailing.rs	71.42% <0.00%> (+14.28%)	⬆️

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[mbr]

Also, I have no idea why changing the readme.md supposedly affects code coverage ¯\(ツ)/¯

[ZoeyR]

Only one minor nit. Otherwise everything looks good! Ignore the code coverage changes, rust is finicky about that.

[ZoeyR]

This last sentence is probably unnecessary since bincode is post 1.0