Update ImageSharp

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Also adds better docs for the new CacheRoot property.

[codecov]

Codecov Report

Merging #130 (09325e1) into master (afff0fe) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #130   +/-   ##
=======================================
  Coverage   88.76%   88.76%           
=======================================
  Files          43       43           
  Lines        1228     1228           
  Branches      146      146           
=======================================
  Hits         1090     1090           
  Misses         99       99           
  Partials       39       39           

Flag	Coverage Δ
unittests	88.76% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...harp.Web/Caching/PhysicalFileSystemCacheOptions.cs	100.00% <ø> (ø)

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