docs: add share-add command

Adds a new command to the docs for sharing a workflow with other users.

Closes reanahub/reana-client#680

docs/reference/reana-client-cli-api/index.md

@@ -37,6 +37,9 @@ Workflow execution commands:
   stop      Stop a running workflow.
   validate  Validate workflow specification file.
 
+Workflow sharing commands:
+  share-add  Share a workflow with other users (read-only).
+
 Workspace interactive commands:
   close  Close an interactive session.
   open   Open an interactive session inside the workspace.
@@ -286,6 +289,23 @@ Examples:
 
      $ reana-client run -w myanalysis-test-big -p myparam=mybigvalue
 
+## Workflow sharing commands
+
+### share-add
+
+Share a workflow with other users (read-only).
+
+The `share-add` command allows sharing a workflow with other users. The
+users will be able to view the workflow but not modify it.
+
+Examples:
+
+<!-- markdownlint-disable no-bare-urls -->
+$ reana-client share-add -w myanalysis.42 --user [email protected]
+
+<!-- markdownlint-disable no-bare-urls -->
+$ reana-client share-add -w myanalysis.42 --user [email protected] --user [email protected] --message "Please review my analysis" --valid-until 2024-12-31
+
 ## Workspace interactive commands
 
 ### open