Add documentation of provided global functions

Author: TysonAndre

README.md

@@ -102,6 +102,48 @@ var_dump($res) //int(5)
 
 ```
 
+## simdjson_php API
+
+```php
+<?php
+/**
+ * Similar to json_decode()
+ *
+ * @returns array|stdClass|string|float|int|bool|null
+ * @throws RuntimeException for invalid JSON (or document over 4GB, or out of range integer/float)
+ */
+function simdjson_decode(string $json, bool $assoc = false, int $depth = 512) {}
+
+/**
+ * Returns true if json is valid.
+ *
+ * @returns ?bool (null if depth is invalid)
+ */
+function simdjson_is_valid(string $json, int $depth = 512) : ?bool {}
+
+/**
+ * Parses $json and returns the number of keys in $json matching the JSON pointer $key
+ *
+ * @returns ?bool (null if depth is invalid)
+ */
+function simdjson_key_count(string $json, string $key, int $depth = 512) : ?int {}
+
+/**
+ * Returns true if the JSON pointer $key could be found.
+ *
+ * @returns ?bool (null if depth is invalid, false if json is invalid or key is not found)
+ */
+function simdjson_key_exists(string $json, string $key, int $depth = 512) : ?bool {}
+
+/**
+ * Returns the value at $key
+ *
+ * @returns array|stdClass|string|float|int|bool|null the value at $key
+ * @throws RuntimeException for invalid JSON (or document over 4GB, or out of range integer/float)
+ */
+function simdjson_key_value(string $json, string $key, bool $assoc = unknown, int $depth = unknown) {}
+```
+
 ## Edge cases
 
 There are some differences from `json_decode()` due to the implementation of the underlying simdjson library. This will throw a RuntimeException if simdjson rejects the JSON.