Add changelog.md

CHANGELOG.md

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+# hls_m3u8
+
+## {next}
+
+ * Performance improvements:
+    + Changed `MediaPlaylist::segments` from `BTreeMap<usize, MediaSegment>`
+      to `StableVec<MediaSegment>`
+    + Added `perf` feature, which can be used to improve performance in the future
+    + Changed all instances of `String` to `Cow<'a, str>` to reduce `Clone`-ing.
+
+ * Most structs now implement [`TryFrom<&'a str>`][TryFrom] instead of [`FromStr`][FromStr].
+
+
+[TryFrom]: https://doc.rust-lang.org/std/convert/trait.TryFrom.html
+[FromStr]: https://doc.rust-lang.org/std/str/trait.FromStr.html