Add faq on how to fix pending issue

docs/admin/mailman.md

+# FAQs
+
+## What if I missed the confirmation email in one specific list?
+Assume I `tommylikehu@gmai.com` missed the confirmation email in list `community@openeuler.org`, the steps would be:
+1. find someone who have admin privilege to mailman resources in the community, now they are (@freesky-edward or @TommyLike).
+2. provide the email address and mail list name.
+Usually the confirmation email would expire in 3 days, but there **MUST** be an issue on that setting, therefore we have to
+fix it via mailman shell by:
+
+    a. login to mailman core node(pod).
+
+    b. login to mail list shell
+    
+    ```
+    mailman shell -l community@openeuler.org:    
+    ```    
+    
+    c. list all pending emails:
+    
+    ```bash
+    util = getUtility(IPendings)
+    list(util.find(mlist=m,pend_type='subscription'))
+    ```
+    d. find out the user's confirmation token by searching the email address, usually the list item would in the format of:
+    ```bash
+    ('hex_token'), {'token_owner': 'subscriber', 'email':
+    'user@example.com', 'display_name': 'Jane User', 'when':
+    '2019-04-09T01:26:08', 'type': 'subscription', 'list_id': 'your.list.id'})
+    ```
+    e. confirm, commit and quit:
+    ```bash
+    util.confirm('hex_token')
+    commit() 
+    ```
+Now we can re-subscribe the mail list on website again.
+
+