update readme, add example [skip ci]

update readme to add an example of writing and reading message from files. fixes #148

PROTOC.md

@@ -79,10 +79,10 @@ The protocol buffers [well known types](https://developers.google.com/protocol-b
 The version of the code included with this package have additional changes to make them compatible with Julia.
 
 You can refer to them in your code after including the following statements:
-````julia
+```julia
 using ProtoBuf
 using ProtoBuf.google.protobuf
-````
+```
 
 While generating code for your `.proto` files that use well-known types, add `ProtoBuf/gen` to the list of includes, e.g.:
 ````julia

USAGE.md

@@ -22,7 +22,7 @@ end
 
 Reading and writing data structures using ProtoBuf is similar to serialization and deserialization. Methods `writeproto` and `readproto` can write and read Julia types from IO streams.
 
-````
+```julia
 julia> using ProtoBuf                       # include protoc generated package here
 
 julia> mutable struct MyType <: ProtoType   # a Julia composite type generated from protoc that
@@ -44,7 +44,72 @@ julia> data.intval
 
 julia> data.strval
 "hello world"
-````
+```
+
+Reading message from a file is very similar to reading from a stream. Here's an example that writes a message to file and then reads it back.
+
+```julia
+julia> include("test_type.jl")
+
+julia> mktemp() do path, io
+           tt1 = TestType(; a="abc", b=true) # construct a message
+           writeproto(io, tt1)  # write message to file
+           close(io) # close the file handle
+           open(path) do io2 # open the file we just wrote in read mode
+               tt2 = readproto(io2, TestType()) # read message from the file
+               @info("read back from file", tt1.a, tt1.b, tt2.a, tt2.b) # print written and read messages
+           end
+       end
+┌ Info: read back from file
+│   tt1.a = "abc"
+│   tt1.b = true
+│   tt2.a = "abc"
+└   tt2.b = true       
+```
+
+Contents of the generated code in test_type.jl:
+
+```julia
+using ProtoBuf
+import ProtoBuf.meta
+
+mutable struct TestType <: ProtoType
+    __protobuf_jl_internal_meta::ProtoMeta
+    __protobuf_jl_internal_values::Dict{Symbol,Any}
+    __protobuf_jl_internal_defaultset::Set{Symbol}
+
+    function TestType(; kwargs...)
+        obj = new(meta(TestType), Dict{Symbol,Any}(), Set{Symbol}())
+        values = obj.__protobuf_jl_internal_values
+        symdict = obj.__protobuf_jl_internal_meta.symdict
+        for nv in kwargs
+            fldname, fldval = nv
+            fldtype = symdict[fldname].jtyp
+            (fldname in keys(symdict)) || error(string(typeof(obj), " has no field with name ", fldname))
+            values[fldname] = isa(fldval, fldtype) ? fldval : convert(fldtype, fldval)
+        end
+        obj
+    end
+end #type TestType
+const __meta_TestType = Ref{ProtoMeta}()
+function meta(::Type{TestType})
+    if !isassigned(__meta_TestType)
+        __meta_TestType[] = target = ProtoMeta(TestType)
+        allflds = Pair{Symbol,Union{Type,String}}[:a => AbstractString, :b => Bool]
+        meta(target, TestType, allflds, [:a], ProtoBuf.DEF_FNUM, ProtoBuf.DEF_VAL, ProtoBuf.DEF_PACK, ProtoBuf.DEF_WTYPES, ProtoBuf.DEF_ONEOFS, ProtoBuf.DEF_ONEOF_NAMES)
+    end
+    __meta_TestType[]
+end
+function Base.getproperty(obj::TestType, name::Symbol)
+    if name === :a
+        return (obj.__protobuf_jl_internal_values[name])::AbstractString
+    elseif name === :b
+        return (obj.__protobuf_jl_internal_values[name])::Bool
+    else
+        getfield(obj, name)
+    end
+end
+```
 
 ## Setting and Getting Fields
 
@@ -59,7 +124,7 @@ Fields that are marked as optional may not be present in an instance of the stru
 - `clear(obj, fld::Symbol)` : clears property `fld` of `obj`.
 - `clear(obj)` : Clears all properties of `obj`.
 
-````
+```julia
 julia> using ProtoBuf
 
 julia> mutable struct MyType <: ProtoType  # a Julia composite type
@@ -87,11 +152,11 @@ julia> readval = readproto(iob, OptType());
 
 julia> hasproperty(readval, :opt)
 false
-````
+```
 
 The `isinitialized(obj::Any)` method checks whether all mandatory fields are set. It is useful to check objects using this method before sending them. Method `writeproto` results in an exception if this condition is violated.
 
-````
+```julia
 julia> using ProtoBuf
 
 julia> import ProtoBuf.meta
@@ -116,7 +181,7 @@ julia> tf.fld1 = TestType(fld1="");
 
 julia> isinitialized(tf)      # true, even though fld2 is not set yet
 true
-````
+```
 
 ## Equality &amp; Hash Value