Create README.md (#2750)

Author: James201311

encoders/README.md

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+# Firebase Encoders
+
+This project provides libraries and code generation infrastructure that allows
+encoding java classes into various target serialization formats(currently
+supported: **json** and **proto**).
+
+The project consists of multiple parts:
+
+*   `firebase_encoders` - Core API and Annotations library.
+*   `processor` - Java plugin that automatically generates encoders for
+    `@Encodable` annotated POJOs.
+*   `firebase_encoders_json` - JSON serialization support.
+*   `firebase_encoders_proto` - Protobuf serialization support.
+*   `protoc_gen` - Protobuf compiler plugin that generates encoder-compliant
+    classes. Can be used with `firebase_encoders_proto` and
+    `firebase_encoders_json`.
+*   `reflective` - Can be used to encode any given class via Java
+    reflection(**not recommented**).
+
+### Protobuf gettings started
+
+##### Step1. Place proto files into **src/main/proto/**
+
+*src/main/proto/my.proto*
+```proto
+syntax = "proto3";
+
+package com.example.my;
+
+import "google/protobuf/timestamp.proto";
+
+message SimpleProto {
+  int32 value = 1;
+  .google.protobuf.Timestamp time = 2;
+}
+```
+
+
+##### Step2. Add the following configurations into gradle module build file.
+
+*example.gradle*
+```gradle
+plugins {
+    id "java-library"
+    id 'com.google.protobuf'
+}
+
+// add a dependency on the protoc plugin's fat jar to make it available to protobuf below.
+configurations.create("protobuild")
+dependencies {
+    protobuild project(path: ":encoders:protoc-gen-firebase-encoders", configuration: "shadow")
+}
+
+protobuf {
+    protoc {
+        artifact = "com.google.protobuf:protoc:$protocVersion"
+    }
+    plugins {
+        firebaseEncoders {
+            path = configurations.protobuild.asPath
+        }
+    }
+    generateProtoTasks {
+        all().each { task ->
+            task.dependsOn configurations.protobuild
+            task.inputs.file 'code-gen-cfg.textproto'
+            task.plugins {
+                firebaseEncoders {
+                    option file('code-gen-cfg.textproto').path
+                }
+            }
+            // In most cases you don't need the full Java output
+            task.builtins {
+                remove java
+            }
+        }
+    }
+}
+
+dependencies {
+    implementation project(":encoders:firebase-encoders")
+    implementation project(":encoders:firebase-encoders-proto")
+    annotationProcessor project(":encoders:firebase-encoders-processor")
+}
+```
+
+##### Step3. Create a code-gen-cfg.textproto file at the module root folder(same location as the gradle module build file).
+
+*code-gen-cfg.textproto* 
+
+Note:
+- The filename must be the same as the filename determined in the gradle build file.
+- Only need to specify the "root" proto object, anything it references will automatically be included.
+```textproto
+# code_gen_cfg.textproto
+# proto-file: src/main/proto/my.proto
+# proto-message: SimpleProto
+
+# all types will be vendored in this package
+vendor_package: "com.google"
+
+# marks a type as a "root" message
+include: "com.example.my.SimpleProto"
+```
+
+With the above configuration here's a list of classes that will be generated:
+
+```
+com.google.com.example.my.SimpleProto
+com.google.com.example.my.SimpleProto$Builder
+com.google.google.protobuf.Timestamp
+com.google.google.protobuf.Timestamp$Builder
+```
+
+Only `root` classes are "encodable" meaning that they have the following
+methods:
+
+```java
+public class SimpleProto {
+  public void writeTo(OutputStream output) throws IOException;
+  public byte[] toByteArray();
+}
+```