Add README and CI

Jenkinsfile

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+pipeline {
+  agent {
+    docker {
+      image 'rust:latest'
+    }
+  }
+  stages {
+    stage('Check') {
+        steps {
+            sh "cargo check"
+        }
+    }
+    stage('Test') {
+        steps {
+            sh "cargo test"
+        }
+    }
+  }
+}

README.md

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+# Rusty Numbers
+
+Playing around with numeric types in Rust.
+