add excon readme (#262)

Author: dmathieu

adapters/excon/README.md

@@ -0,0 +1,50 @@
+# Excon Adapter
+
+The excon adapter is a community-maintained instrumentation adapter for the [excon][excon-home] HTTP library.
+
+## How do I get started?
+
+Install the gem using:
+
+```
+gem install opentelemetry-adapters-excon
+```
+
+Or, if you use [bundler][bundler-home], include `opentelemetry-adapters-excon` to your `Gemfile`.
+
+## Usage
+
+To install the instrumentation adapter, call `use` with the name of the instrumentation.
+
+
+```ruby
+OpenTelemetry::SDK.configure do |c|
+  c.use 'OpenTelemetry::Adapters::Excon'
+end
+```
+
+Alternatively, you can also call `use_all` to install all the available adapters.
+
+```ruby
+OpenTelemetry::SDK.configure do |c|
+  c.use_all
+end
+```
+
+## How can I get involved?
+
+The `opentelemetry-adapters-excon` gem source is [on github][repo-github], along with related gems including `opentelemetry-api` and `opentelemetry-sdk`.
+
+The OpenTelemetry Ruby gems are maintained by the OpenTelemetry-Ruby special interest group (SIG). You can get involved by joining us on our [gitter channel][ruby-gitter] or attending our weekly meeting. See the [meeting calendar][community-meetings] for dates and times. For more information on this and other language SIGs, see the OpenTelemetry [community page][ruby-sig].
+
+## License
+
+The `opentelemetry-adapters-all` gem is distributed under the Apache 2.0 license. See [LICENSE][license-github] for more information.
+
+[excon-home]: https://github.com/excon/excon
+[bundler-home]: https://bundler.io
+[repo-github]: https://github.com/open-telemetry/opentelemetry-ruby
+[license-github]: https://github.com/open-telemetry/opentelemetry-ruby/blob/master/LICENSE
+[ruby-sig]: https://github.com/open-telemetry/community#ruby-sig
+[community-meetings]: https://github.com/open-telemetry/community#community-meetings
+[ruby-gitter]: https://gitter.im/open-telemetry/opentelemetry-ruby

adapters/excon/opentelemetry-adapters-excon.gemspec

@@ -21,7 +21,7 @@ Gem::Specification.new do |spec|
 
   spec.files = ::Dir.glob('lib/**/*.rb') +
                ::Dir.glob('*.md') +
-               ['LICENSE']
+               ['README.md', 'LICENSE']
   spec.require_paths = ['lib']
   spec.required_ruby_version = '>= 2.5.0'