Fix: add option overwriteList

Due to MaxMind mmdb file format constraint,
the IPs and/or CIDRs of the latter written list
will overwrite those of the former one
when duplicated data is found.

To make sure that the lists you added include
all their own IPs and/or CIDRs, you must place
the name of the most important list at last
in option wantedList and option overwriteList
of type maxmindMMDB in config file.

The option overwriteList of type maxmindMMDB
is used when the IPs and/or CIDRs of some lists
in generated mmdb file overlap.

For example, if you change the CIDRs of the list "cn",
and generate the full version of Country.mmdb file
which is with all countries(lists), the option overwriteList
must include "cn", so that the "cn" list will be written at last,
which ensures it includes all the CIDRs you specify.

If the option wantedList of type maxmindMMDB is specified,
no need for option overwriteList.

1 files changed, 6 insertions, 0 deletions

diff --git a/README.md b/README.md
index 795cde93..0b54cc06 100644
--- a/README.md
+++ b/README.md
@@ -167,6 +167,12 @@ These two concepts are notable: `input` and `output`. The `input` is the data so
 - **clashRuleSet**：[ipcidr 类型的 Clash RuleSet](https://github.com/Dreamacro/clash/wiki/premium-core-features#ipcidr)
 - **surgeRuleSet**：[Surge RuleSet](https://manual.nssurge.com/rule/ruleset.html)
 
+### 注意事项
+
+由于 MaxMind mmdb 文件格式的限制，当不同列表的 IP 或 CIDR 数据有交集或重复项时，后写入的列表的 IP 或 CIDR 数据会覆盖（overwrite）之前已写入的列表的数据。譬如，IP `1.1.1.1` 同属于列表 `AU` 和列表 `Cloudflare`。如果 `Cloudflare` 在 `AU` 之后写入，则 IP `1.1.1.1` 归属于列表 `Cloudflare`。
+
+为了确保某些指定的列表、被修改的列表一定囊括属于它的所有 IP 或 CIDR 数据，可在 `output` 输出格式为 `maxmindMMDB` 的配置中增加选项 `overwriteList`，该选项中指定的列表会在最后逐一写入，列表中最后一项优先级最高。若已设置选项 `wantedList`，则无需设置 `overwriteList`。`wantedList` 中指定的列表会在最后逐一写入，列表中最后一项优先级最高。
+
 ## CLI 功能展示
 
 可通过 `go install -v github.com/Loyalsoldier/geoip@latest` 直接安装 CLI。