Add cycle field to sat extension

CHANGELOG.md

@@ -8,6 +8,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- Added cycle field ([#6](https://github.com/stac-extensions/sat/issues/6))
+
 ### Changed
 
 ### Deprecated

README.md

@@ -28,7 +28,7 @@ It will often be combined with other extensions that describe the actual data, s
 | sat:absolute_orbit                    | integer | The absolute orbit number at the time of acquisition.                                                                                                                                                |
 | sat:relative_orbit                    | integer | The relative orbit number at the time of acquisition.                                                                                                                                                |
 | sat:anx_datetime                      | string  | The [Ascending Node](https://en.wikipedia.org/wiki/Orbital_node) Crossing (ANX) time, in UTC. It is formatted according to [RFC 3339, section 5.6](https://tools.ietf.org/html/rfc3339#section-5.6). |
-| sat:cycle                             | integer | The repeat cycle of the satellite.                                                                                                                                                                   |
+| sat:cycle                             | integer | The number of repeat cycle done by the satellite at the time of the acquisition. [Repeat cycle](https://ltb.itc.utwente.nl/page/498/concept/81577) is the time between two successive identical orbits.                                                                                                                                                                   |
 
 *At least one of the fields must be specified.*