Add a Using the library section to README and list the ESP-IDF build's dependencies in NOTICES.md #40

Problem

README has no snippet showing start, a listener, open_input() and a write loop, and no CMake consumption snippet; the only example is the 2.5k-line server_cli. NOTICES.md omits the ESP-IDF build's dependencies (the esphome/noise-c component, libsodium, esp-dsp, mbedTLS) and gives no upstream URL for micro-resampler.

Evidence

Checked against the code at e69f3b46a801:

• README.md has 111 lines and no code snippet showing start/provider/listener/open_input plus a write loop, and no CMake consumption (add_subdirectory or FetchContent) snippet. The only build commands are for the library's own build, at lines 28-46.
• ESP-IDF gets one line (README.md:46): 'The library also builds as an ESP-IDF component (cmake/esp_idf.cmake).'
• Line counts match. examples/server_cli totals 7474 lines, main.cpp has 2496, web_assets/dashboard.html has 738, and run() starts at line 1987 and runs to the end of the file, about 500 lines.
• NOTICES.md omits the ESP build's dependencies. cmake/esp_idf.cmake:7-15 requires esphome__noise-c, esphome__libsodium, espressif__esp-dsp and mbedtls, and NOTICES.md lists libsodium, esp-dsp and mbedTLS nowhere. noise-c is listed only as the host's fetched esphome-libs fork, with no note on the ESP component.
• NOTICES.md's micro-resampler entry has no upstream URL. It gives Location, Version, License and Copyright, whereas the stb entry has a 'Source:' URL. third_party/micro-resampler/VENDORED.md has none either.

Checked and not part of this issue

• docs/conventions.md:66 says server_cli 'uses only the public API' and that is true. Every sendspin include in examples/server_cli is <sendspin/server/{server,config,types,discovery}.h>, and none reaches into src/. Linking sendspin_server_cpp_beast (examples/CMakeLists.txt:30) is for server_cli's own --web dashboard (web_ui.cpp on Beast, per the comment at lines 28-29). It is a third-party dependency of the example, not a use of a non-public library API. The convention is not misleading, so this sub-claim should be dropped.
• 'Relies on the archive's Asio definitions' is a build-linkage detail. The beast target is an INTERFACE target carrying the compile definitions (cmake/dependencies.cmake:34-45) that any consumer including Beast needs. It is not an API-boundary violation.
• The 'a first user must read 2.5k lines' framing is overstated. include/sendspin/server/server.h carries the threading contract and doc comments, and README.md:66-108 documents server_cli's commands and options. A snippet would still help, but the header docs are a partial substitute.

Direction

Add the section with both snippets, and complete NOTICES.md.

Done when

• README shows the minimal use; NOTICES.md lists every ESP-IDF dependency.