Document how the Monero iOS library is built

apps/ios/Monero/toolchain/depends/hosts/ios.mk

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+# iOS host for monero depends system (arm64 simulator for dev).
+# Targets modern iOS (14+) on Apple Silicon simulators / devices.
+
+IOS_MIN_VERSION=13.0
+LD64_VERSION=609
+
+ifndef IOS_PLATFORM
+IOS_PLATFORM=SIMULATOR
+endif
+
+ifeq ($(IOS_PLATFORM),SIMULATOR)
+IOS_SDK_ROOT := $(shell xcrun --sdk iphonesimulator --show-sdk-path)
+CC_target=aarch64-apple-ios-simulator
+else
+IOS_SDK_ROOT := $(shell xcrun --sdk iphoneos --show-sdk-path)
+CC_target=aarch64-apple-ios
+endif
+
+ios_CC=clang -target $(CC_target) -miphoneos-version-min=$(IOS_MIN_VERSION) --sysroot $(IOS_SDK_ROOT) -mlinker-version=$(LD64_VERSION)
+ios_CXX=clang++ -target $(CC_target) -miphoneos-version-min=$(IOS_MIN_VERSION) --sysroot $(IOS_SDK_ROOT) -mlinker-version=$(LD64_VERSION) -stdlib=libc++
+
+ios_CFLAGS=-pipe -isysroot $(IOS_SDK_ROOT)
+ios_CXXFLAGS=-pipe -isysroot $(IOS_SDK_ROOT) -std=c++14 -stdlib=libc++
+ios_ARFLAGS=cr
+
+ios_release_CFLAGS=-O1
+ios_release_CXXFLAGS=$(ios_release_CFLAGS)
+
+ios_debug_CFLAGS=-O1
+ios_debug_CXXFLAGS=$(ios_debug_CFLAGS)
+
+ios_native_toolchain=native_cctools darwin_sdk
+
+# iOS uses the system (macOS) ar/ranlib/strip/nm for cross-compiled archives.
+ios_AR=ar
+ios_RANLIB=ranlib
+ios_STRIP=strip
+ios_NM=nm
+ios_LIBTOOL=libtool
+ios_OTOOL=otool
+ios_INSTALL_NAME_TOOL=install_name_tool