add: Add README.md

Author: Awayume

README.md

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+# cfddns (Cloudflare DDNS tool)
+This is a script to use Cloudflare as DDNS.  
+This works on Linux only.
+
+## How to use
+
+### 1. Get a Cloudflare Zone ID and a Service key
+Access your Cloudflare dashboard of your domain.  
+On "API" section, you can find "Zone ID".  
+Next, access [API Tokens page](https://dash.cloudflare.com/profile/api-tokens), and create new API Token.  
+You should grant permissions "Zone/DNS/Read" and "Zone/DNS/Edit".  
+Also, you should add your domain to "Zone Resources".  
+I recommend select "Include/Specific zone/[domain]" because token is saved on the text file.  
+
+### 2. Edit configuration file
+The configuration file format is written below.
+
+### 3. Execute cfddns
+You should install Python 3 to execute cfddns.  
+You can execute cfddns with command line, but I recommend use systemd.  
+The example is written below.
+
+## Configuration file format
+```
+# cfddns config                                 
+zone_id=[Zone ID]
+service_key=[API Token]
+domains=[a.example.com],[b.example.com]
+ipv6=[yes/no]
+```
+
+## Example of systemd unit file
+```
+[Unit]
+Description = Cloudflare DDNS Service
+
+[Service]
+ExecStart = /usr/local/sbin/cfddns
+Restart = always
+Type = simple
+StandardOutput = journal
+StandardError = journal
+
+[Install]
+WantedBy = multi-user.target
+```
+
+## Known issues
+- If domain is not A or AAAA record, cfddns will crash.
+- If record has other records (ex. MX, TXT) as same name, cfddns will crash.
+- Is is not able to set both A and AAAA record.
+- Only one zone can be set.
+- Configuration file format is too strict.