Update changelog.md (#525)

* Update changelog.md

We've been making changes without updating the changelog. This summarizes the changes since v7.14 was released.

* Update changelog.md

* Update changelog.md

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Co-authored-by: Dave Thaler <[email protected]>

changelog.md

@@ -1,3 +1,22 @@
+# Main
+
+- Clarified that `FORM`.`MEDI` describe the original medium, not the derived medium, when used with derived files.
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+- Clarified the meaning of the `WWW` structure, which previously only mentioned its payload datatype.
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+- Clarified `PLAC` to both define "jurisdiction" and document its meaning in the absence of a `PLAC`.`FORM`.
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+- Clarified what the term "principal date" means in different contexts in the definition of `g7:DATE`.
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+- Updated `NICK` to no longer suggest that some names are "improper" and to document the diversity of views in what a "nickname" is.
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+- Removed confusing reference to superstructures in the meaning of a documented extension tag.
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+- Added ABNF for more datatypes and updated DIGIT's capitalization for compatibility with more ABNF toolchains.
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+- Various typo corrections.
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+
 # Version 7.0.14
 
 - Recommend that `NO XYZ` only be used where `XYZ` is permitted (its meaning is undefined elsewhere).