docs(settings.rst): add 'timestamp-negative' to available parsers

docs/settings.rst

@@ -144,7 +144,7 @@ Language Detection
 Default Languages
 +++++++++++++++++
 
-``DEFAULT_LANGUAGES``: It is a ``list`` of language codes in ISO 639 that will be used as default 
+``DEFAULT_LANGUAGES``: It is a ``list`` of language codes in ISO 639 that will be used as default
 languages for parsing when language detection fails. eg. ["en", "fr"]:
 
     >>> from dateparser import parse
@@ -181,6 +181,9 @@ The following parsers exist:
     followed by additional digits or a period (``.``), those first 10 digits
     are interpreted as `Unix time <https://en.wikipedia.org/wiki/Unix_time>`_.
 
+-    ``'negative-timestamp'``: It is similar to ``'timestamp'``, but parses the
+    negative timestamp.
+
 -   ``'relative-time'``: Parses dates and times expressed in relation to the
     current date and time (e.g. “1 day ago”, “in 2 weeks”).