document minimum nginx version

[bmw]

it seems nginx>=1.25.0 (and in particular the commit nginx/nginx@9e489d2) is needed in order to build this module. without it, i get the following error when trying to compile:

error[E0425]: cannot find function `ngx_http_validate_host` in crate `nginx_sys`
   --> src/conf/order.rs:252:41
    |
252 |     let rc = Status(unsafe { nginx_sys::ngx_http_validate_host(&mut host, pool.as_mut(), 1) });
    |                                         ^^^^^^^^^^^^^^^^^^^^^^ not found in `nginx_sys`

For more information about this error, try `rustc --explain E0425`.
error: could not compile `nginx-acme` (lib) due to 1 previous error

might it be worth documenting this in the readme somewhere?

(i'm loving this project by the way and am very impressed with it so far. nice work 👏 )

[hzbd]

min develop version: NGINX 1.25.1 
min stable version: NGINX 1.26.1 
current stable version: NGINX 1.28.0

[hzbd]

Here is an article in Chinese on how to use NGINX ACME.

https://sconts.com/post/nginx-native-acme-support/