Document library build/install and basic usage

Improves jaegertracing#33

Documents jaegertracing#38, jaegertracing#45, jaegertracing#47, jaegertracing#51

Signed-off-by: Craig Ringer <craig@2ndquadrant.com>

README.md

@@ -3,6 +3,181 @@
 # cpp-client
 C++ OpenTracing binding for Jaeger
 
+jaeger-cpp is a client library to emit trace information for the Jaeger trace
+collector <http://jaeger.readthedocs.io>.
+
+It's written in C++11 and implements the OpenTracing API; see [ot-url] and
+https://github.com/opentracing/opentracing-cpp.
+
+This library is just an agent. It sends UDP packets to a trace collector, which
+does all the real work. Agent libraries also exist for other languages like
+Java and Go.
+
+## Quick build
+
+To do a test build and install in $HOME/jaeger-cpp-client:
+
+    git clone https://github.com/jaegertracing/cpp-client.git jaeger-cpp-client
+    mkdir jaeger-cpp-build
+    cd jaeger-cpp-build
+    cmake -DCMAKE_INSTALL_PREFIX=$HOME/jaeger-cpp-client ../jaeger-cpp-client
+    make
+    make install
+
+## Usage
+
+To use jaeger-cpp in your code you mostly use the OpenTracing C++ APIs. Your code
+must instantiate and configure the Jaeger tracer, but the rest is the same as for
+any other OpenTracing backend.
+
+The simplest way to instantiate and configure a Jaeger tracer is to use the
+yaml configuration support to create the tracer. Then set it as the OpenTracing
+global tracer. Typically a caller would read the configuration in from a file,
+but as an example:
+
+    #include <jaegertracing/Tracer.h>
+
+    void initTracing()
+    {
+        constexpr auto kConfigYAML = R"cfg(
+            disabled: false
+            sampler:
+                type: const
+                param: 1
+            reporter:
+                queueSize: 100
+                bufferFlushInterval: 10
+                logSpans: false
+                localAgentHostPort: 127.0.0.1:6831
+            headers:
+                jaegerDebugHeader: debug-id
+                jaegerBaggageHeader: baggage
+                TraceContextHeaderName: trace-id
+                traceBaggageHeaderPrefix: "testctx-"
+            baggage_restrictions:
+                denyBaggageOnInitializationFailure: false
+                hostPort: 127.0.0.1:5778
+                refreshInterval: 60
+            )cfg";
+
+        const auto config = jaegertracing::Config::parse(YAML::Load(kConfigYAML));
+        auto tracer = jaegertracing::Tracer::make("postgresql", config);
+        opentracing::Tracer::InitGlobal(tracer);
+    }
+
+    void teardownTracing()
+    {
+        opentracing::Tracer::Global()->Close
+    }
+
+A C++-language configuration facility is also-available; see
+`jaegertracing/Config.h` and the jaeger-cpp tests. A trivial example:
+
+    auto config = jaegertracing::Config(
+        false,
+        jaegertracing::samplers::Config(jaegertracing::kSamplerTypeConst, 1),
+        jaegertracing::reporters::Config(
+            jaegertracing::reporters::Config::kDefaultQueueSize,
+            std::chrono::seconds(1), true));
+
+
+Once instantiated, simple traces may be performed with the usual opentracing
+facilities e.g.:
+
+    #include <chrono>
+    #include <thread>
+
+    int main()
+    {
+        initTracing();
+
+        auto tracer = opentracing::Tracer::Global();
+        auto span = tracer->StartSpan("abc");
+        std::this_thread::sleep_for(std::chrono::milliseconds{10});
+        span->Finish();
+        std::this_thread::sleep_for(std::chrono::seconds{5});
+
+        teardownTracing();
+    }
+
+The Jaeger tracing instance usually lives as long as the traced process.
+
+The tracer should be closed before program exit so it can flush its buffers,
+so traces aren't lost.
+
+### Usage from C
+
+It's possible to use `jaeger-cpp` from C with appropriate `extern "C"` thunks
+and care around resource lifetime management etc. A real world example is the
+support for opentracing and jaeger in ngnix; see
+https://github.com/opentracing-contrib/nginx-opentracing . A simplified example
+would be welcomed.
+
+### Usage from C++98
+
+No examples currently exist. It'll at least be possible to do so via an "extern
+C" thunking layer and C-style coding.
+
+## Dependencies
+---
+
+Compiling jaeger-cpp requires:
+
+* opentracing-cpp <https://github.com/opentracing/opentracing-cpp>
+* nlohmann json (2.1.0 or greater) <https://github.com/nlohmann/json>
+* yaml-cpp <https://github.com/jbeder/yaml-cpp>
+* boost regex <http://www.boost.org>
+* Apache thrift <https://thrift.apache.org/>
+* Google Test <https://github.com/google/googletest>
+
+### Automatic dependencies - Hunter
+
+By default, Jaeger uses Hunter <http://hunter.sh> to download and build
+required dependencies (including opentracing) from the Internet as part of the
+CMake build preparation.
+
+Hunter does not install these dependencies as part of `jaeger-cpp`'s `make
+install` target, so applications that wish to use a Hunter-built jaeger-cpp
+should also use Hunter.
+
+### Locally installed dependencies
+
+To perform a traditional installation to the local system, install all the
+above dependencies, then build jaeger-cpp with `-DHUNTER_ENABLED=0` and
+`make install` as usual.
+
+Specify the install directory with the `CMAKE_INSTALL_PREFIX` cmake variable.
+The default is platform dependent, but `/usr/local` on most UNIXes.
+
+#### Fedora
+
+These dependencies are all in Fedora 25+ except for nlohmann json 2.1.x.
+
+Install with:
+
+    sudo dnf install yaml-cpp-devel boost-devel thrift-devel gtest-devel
+
+There's a nlohmann json package `json-cpp` too, but it's too old so you'll
+have to compile it yourself.
+
+## Build options
+
+CMake flags available for setting with `-D`:
+
+* `HUNTER_ENABLED`: set to 0 to disable the use of the Hunter dependency
+  fetching/building tool to download jaeger-cpp's dependencies. Enabled by
+  defualt.
+
+* `BUILD_TESTING`: set to 0 to disable test compilation
+
+* `JAEGERTRACING_WITH_YAML_CPP`: disable yaml configuration support
+
+* `CMAKE_INSTALL_PREFIX`: base directory to install `jaeger-cpp`
+
+Most other cmake standard variables
+<https://cmake.org/cmake/help/latest/manual/cmake-variables.7.html> are
+supported.
+
 [ci-img]: https://travis-ci.org/jaegertracing/cpp-client.svg?branch=master
 [ci]: https://travis-ci.org/jaegertracing/cpp-client
 [cov-img]: https://codecov.io/gh/jaegertracing/cpp-client/branch/master/graph/badge.svg