Update README.md

Author: dragon7307

src/README.md

@@ -43,6 +43,38 @@ The following **Extension Methods** are contained:
 - `(ReadOnly-)Span<T>.Takelast(int count)`
 - `(ReadOnly-)Span<T>.SkipWhile(Predicate<T> condition)` 
 - `(ReadOnly-)Span<T>.TakeWhile(Predicate<T> condition)`
+- `(Readonly-)Span<T>.First()`
+- `(Readonly-)Span<T>.First(Predicate<T> predicate)`
+- `(Readonly-)Span<T>.FirstOrDefault()`
+- `(Readonly-)Span<T>.FirstOrDefault(Predicate<T> predicate)`
+- `(Readonly-)Span<T>.FirstOrDefault(T defaultValue)`
+- `(Readonly-)Span<T>.FirstOrDefault(Predicate<T> predicate, T defaultValue)`
+- `(Readonly-)Span<T>.Last()`
+- `(Readonly-)Span<T>.Last(Predicate<T> predicate)`
+- `(Readonly-)Span<T>.LastOrDefault()`
+- `(Readonly-)Span<T>.LastOrDefault(Predicate<T> predicate)`
+- `(Readonly-)Span<T>.LastOrDefault(T defaultValue)`
+- `(Readonly-)Span<T>.LastOrDefault(Predicate<T> predicate, T defaultValue)`
+- `(Readonly-)Span<T>.Single()`
+- `(Readonly-)Span<T>.Single(Predicate<T> predicate)`
+- `(Readonly-)Span<T>.SingleOrDefault()`
+- `(Readonly-)Span<T>.SingleOrDefault(Predicate<T> predicate)`
+- `(Readonly-)Span<T>.SingleOrDefault(T defaultValue)`
+- `(Readonly-)Span<T>.SingleOrDefault(Predicate<T> predicate, T defaultValue)`
+- `(Readonly-)Span<T>.ElementAt(int index)`
+- `(Readonly-)Span<T>.ElementAt(Index index)`
+- `(Readonly-)Span<T>.ElementAtOrDefault(int index)`
+- `(Readonly-)Span<T>.ElementAtOrDefault(Index index)`
+- `(Readonly-)Span<T>.ElementAtOrDefault(int index, T defaultValue)`
+- `(Readonly-)Span<T>.ElementAtOrDefault(Index index, T defaultValue)`
+- `(Readonly-)Span<T>.Min()`
+- `(Readonly-)Span<T>.Min(Func<TSource, TResult> selector)`
+- `(Readonly-)Span<T>.MinBy(Func<TSource, TKey> keySelector)`
+- `(Readonly-)Span<T>.MinBy(Func<TSource, TKey> keySelector, IComparer<TKey> comparer)`
+- `(Readonly-)Span<T>.Max()`
+- `(Readonly-)Span<T>.Max(Func<TSource, TResult> selector)`
+- `(Readonly-)Span<T>.MaxBy(Func<TSource, TKey> keySelector)`
+- `(Readonly-)Span<T>.MaxBy(Func<TSource, TKey> keySelector, IComparer<TKey> comparer)`
 
 ## Contributing