Update README.md

- Clarify the use of the default `keyid`

README.md

@@ -34,7 +34,7 @@ flowchart TD
 |`description`|`string`|Human readable description of the set.|✓|
 |`generated-timestamp`|`number`|Generation timestamp in epoch format. |✓|
 |`algorithm`|`string`|Keyed-hash message authentication. Available:<br/> - Blake2b<br/> - Blake3<br/> - HMAC-SHA-256<br /> - HMAC-SHA-512  |&check;|
-|`keyid`|`string`|The reference to the key used in the keyed-hash message authentication algorithm.|&check;|
+|`keyid`|`string`|The reference to the key used in the keyed-hash message authentication algorithm. If the default value is used, then the private shared key `infected`.|&check;|
 |`bloomfilter`|`hash`|The BloomFilter description along with its format and model.|&check;|
 |`misp-attribute-types`|`array`|Array of `string` with the types covered by the private search set. Types can be any from types [mentioned in the default MISP types](https://www.circl.lu/doc/misp/categories-and-types/#types). If not specified, `text` type is covered.|-|
 |`misp-object-template`|`array`|Array of `string` with the object template name and the version separated with a semicolon such as `person:19`.|-|