docs: add commit_stats docs

Signed-off-by: Miguel Molina <[email protected]>

Author: erizocosmico

CHANGELOG.md

@@ -6,6 +6,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Added documentation about `commit_stats`.
+
+### Changed
+
+- internal/function: gracefully handle errors in commit_stats.
+
 ### Fixed
 
 - internal/function: take into account if repository is resolved in commit_stats ([#863](https://github.com/src-d/gitbase/pull/863))

docs/using-gitbase/functions.md

@@ -6,6 +6,7 @@ To make some common tasks easier for the user, there are some functions to inter
 
 |     Name     |                                               Description                                                                      |
 |:-------------|:-------------------------------------------------------------------------------------------------------------------------------|
+|`commit_stats(repository_id, [from_commit_hash], to_commit_hash)`|returns the stats between two commits for a repository. If from is empty, it will compare the given `to_commit_hash` with its parent commit|
 |`is_remote(reference_name)bool`| check if the given reference name is from a remote one                                                          |
 |`is_tag(reference_name)bool`| check if the given reference name is a tag                                                                         |
 |`is_vendor(file_path)bool`| check if the given file name is a vendored file                                                                         |