Add OPTINS_SHARE_ALL_ORGS to env doc

Author: engelhartrueben

docs/REFERENCE-environment_variables.md

@@ -88,6 +88,7 @@
 | OPT_OUT_MESSAGE                                  | Spoke instance-wide default for opt out message.                                                                                                                                                                                                                                                                                                                                            |
 | OPT_OUT_PER_STATE                                | Have different opt-out messages per state and org. Defaults to the organization's default opt-out message for non-specified states or when the Smarty Zip Code API is down. Requires the `SMARTY_AUTH_ID` and `SMARTY_AUTH_TOKEN` environment variables. |
 | OPTOUTS_SHARE_ALL_ORGS                           | Can be set to true if opt outs should be respected per instance and across organizations                                                                                                                                                                                                                                                                                                    |
+| OPTINS_SHARE_ALL_ORGS |  Can be set to true if opt ins should be respected per instance and across organizations |
 | OUTPUT_DIR                                       | Directory path for packaged files should be saved to. _Required_.                                                                                                                                                                                                                                                                                                                           |
 | OWNER_CONFIGURABLE                               | If set to `ALL` then organization owners will be able to configure all available options from their Settings section (otherwise only superadmins will). You can also put a comma-separated list of environment variables to white-list specific settable variables here.  This gives organization owners a lot of control of internal settings, so enable at your own risk.                 |
 | PASSPORT_STRATEGY                                | A flag to set passport strategy to use for user authentication. The Auth0 strategy will be used if the value is an empty string or `auth0`. The local strategy will be used if the value is `local`.                                                                                                                                                                                        |