Add readme for mailchimp input (#6210)

plugins/inputs/mailchimp/README.md

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+# Mailchimp Input
+
+Pulls campaign reports from the [Mailchimp API](https://developer.mailchimp.com/).
+
+### Configuration
+
+This section contains the default TOML to configure the plugin.  You can
+generate it using `telegraf --usage mailchimp`.
+
+```toml
+[[inputs.mailchimp]]
+  ## MailChimp API key
+  ## get from https://admin.mailchimp.com/account/api/
+  api_key = "" # required
+  
+  ## Reports for campaigns sent more than days_old ago will not be collected.
+  ## 0 means collect all and is the default value.
+  days_old = 0
+  
+  ## Campaign ID to get, if empty gets all campaigns, this option overrides days_old
+  # campaign_id = ""
+```
+
+### Metrics
+
+- mailchimp
+  - tags:
+    - id
+    - campaign_title
+  - fields:
+    - emails_sent (integer, emails)
+    - abuse_reports (integer, reports)
+    - unsubscribed (integer, unsubscribes)
+    - hard_bounces (integer, emails)
+    - soft_bounces (integer, emails)
+    - syntax_errors (integer, errors)
+    - forwards_count (integer, emails)
+    - forwards_opens (integer, emails)
+    - opens_total (integer, emails)
+    - unique_opens (integer, emails)
+    - open_rate (double, percentage)
+    - clicks_total (integer, clicks)
+    - unique_clicks (integer, clicks)
+    - unique_subscriber_clicks (integer, clicks)
+    - click_rate (double, percentage)
+    - facebook_recipient_likes (integer, likes)
+    - facebook_unique_likes (integer, likes)
+    - facebook_likes (integer, likes)
+    - industry_type (string, type)
+    - industry_open_rate (double, percentage)
+    - industry_click_rate (double, percentage)
+    - industry_bounce_rate (double, percentage)
+    - industry_unopen_rate (double, percentage)
+    - industry_unsub_rate (double, percentage)
+    - industry_abuse_rate (double, percentage)
+    - list_stats_sub_rate (double, percentage)
+    - list_stats_unsub_rate (double, percentage)
+    - list_stats_open_rate (double, percentage)
+    - list_stats_click_rate (double, percentage)