support `EXIF_TAG_USER_COMMENT`

[t-bltg]

Tests added, adding EXIF_TAG_MAKER_NOTE is left for another PR.

I also took the liberty of updating some docstrings, and the README.md.

Fix https://github.com/JuliaImages/ExifViewer.jl/issues/24.

[codecov[bot]]

Codecov Report

Patch coverage: 93.33% and project coverage change: +0.20 :tada:

Comparison is base (476fb7a) 90.21% compared to head (d23f7de) 90.42%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #25 +/- ## ========================================== + Coverage 90.21% 90.42% +0.20% ========================================== Files 5 5 Lines 184 188 +4 ========================================== + Hits 166 170 +4 Misses 18 18 ``` | [Impacted Files](https://app.codecov.io/gh/JuliaImages/ExifViewer.jl/pull/25?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaImages) | Coverage Δ | | |---|---|---| | [src/write.jl](https://app.codecov.io/gh/JuliaImages/ExifViewer.jl/pull/25?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaImages#diff-c3JjL3dyaXRlLmps) | `89.36% <93.33%> (+0.47%)` | :arrow_up: |

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[ashwani-rathee]

BTW is writing EXIF_TAG_MAKER_NOTE important for you? We can read them right now just fine but writing them is still needed to be done.

[t-bltg]

BTW is writing EXIF_TAG_MAKER_NOTE important for you? We can read them right now just fine but writing them is still needed to be done.

Not at the moment, but I though that integrating more tags was always better. This is left fo another PR :)