Create README.md with a few examples

(cherry picked from commit 232cc80)

camunda-sdk-java/java-client-operate/README.md

@@ -0,0 +1,58 @@
+# Java Operate client
+
+## Build the client
+
+### SaaS Authentication
+
+```java
+  JwtConfig jwtConfig = new JwtConfig();
+  jwtConfig.addProduct(Product.OPERATE, new JwtCredential(clientId, clientSecret));
+  targetOperateUrl = "https://" + region + ".operate.camunda.io/" + clusterId;
+  auth = SaaSAuthentication.builder().jwtConfig(jwtConfig).build();
+
+  client = CamundaOperateClient.builder()
+              .operateUrl(targetOperateUrl)
+              .authentication(auth)
+              .setup()
+              .build();
+```
+
+### SelfManaged Authentication
+
+```java
+  JwtConfig jwtConfig = new JwtConfig();
+  jwtConfig.addProduct(Product.OPERATE, new JwtCredential(clientId, clientSecret));
+  auth = SelfManagedAuthentication.builder().jwtConfig(jwtConfig).keycloakUrl(keycloakUrl).build();
+
+  client = CamundaOperateClient.builder()
+              .operateUrl(operateUrl)
+              .authentication(auth)
+              .setup()
+              .build();
+```
+
+## Use the client
+
+### List deployed process definitions
+
+```java
+  ProcessDefinitionFilter processDefinitionFilter = ProcessDefinitionFilter.builder().build();
+  SearchQuery procDefQuery = new SearchQuery.Builder()
+            .filter(processDefinitionFilter)
+            .size(1000)
+            .sort(new Sort("version", SortOrder.DESC))
+            .build();
+  return camundaOperateClient.searchProcessDefinitions(procDefQuery);
+```
+
+### Read process definitions content
+
+```java
+  camundaOperateClient.getProcessDefinitionXml(ProcessDefinitionKey);
+```
+
+### List variables
+
+```java
+  return camundaOperateClient.searchVariables(new SearchQuery.Builder().filter(new VariableFilter()).size(100).build());
+```