doc: add some infos

- add some infos in doc
- add count limit prompt in query step

legacy/README.md

@@ -1,5 +1,51 @@
 # LCR Connector (Legacy)
 
-在3个元器件内，使用给定元器件数值列表快速找到目标数值元器件的最好拼接方式，支持电阻，电容，电感
+Get the resistor, capacitor, or inductor circuit which has the closest value for your given value within at most 3 devices.
 
-执行`uv run lcr-connector --help`来查阅参数手册。
+This is the legacy version of LCR Connector, although this is also refactored from true legacy version in modern Python.
+
+## Usage
+
+- Execute `uv sync` to configure the environment.
+- Execute `uv run lcr-connector --help` for the usage of LCR Connector.
+- After launch LCR Connector, you can see the help message in interactive console, or type `help` to see the help message.
+
+Additionaly, for using LCR Connector, you need 3 list files holding all possible device standard values which are available in your laboratory.
+Each of them represents a type of device respectively, resistor, capacitor, or inductor.
+These list files are basically like this:
+
+```
+100
+220
+270
+390
+470
+680
+1k
+1.2k
+1.5k
+2.2k
+3.3k
+4.7k
+6.8k
+10k
+47k
+100k
+1M
+```
+
+Supported units are:
+
+- n: Nano
+- p: Pico
+- u: Micro
+- m: Milli
+- k: Kilo
+- M: Mega
+- G: Giga
+
+There is no physical unit for the values in the list files.
+
+Unit is optional. If you don't specify a unit, the value is considered as a plain floating value.
+
+Unit is **case sensitive** to distinguish between milli and mega (e.g. 1m is milli, 1M is mega).