Started writing documentation

git-svn-id: https://svn.kapsi.fi/jpa/nanopb@953 e3a754e5-d11d-0410-8d38-ebb782a927b9

docs/Makefile

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+all: index.html
+
+%.html: %.rst
+	rst2html --stylesheet=lsr.css --link-stylesheet $< $@

docs/index.rst

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+=============================================
+Nanopb: Protocol Buffers with small code size
+=============================================
+
+Nanopb is an ANSI-C library for encoding and decoding messages in Google's `Protocol Buffers`__ format with minimal requirements for RAM and code space.
+It is primarily suitable for 32-bit microcontrollers.
+
+__ http://code.google.com/apis/protocolbuffers/
+
+Overall structure
+=================
+
+For the runtime program, you always need *pb.h* for type declarations.
+Depending on whether you want to encode, decode or both, you also need *pb_encode.h/c* or *pb_decode.h/c*.
+
+The high-level encoding and decoding functions take an array of *pb_field_t* structures, which describes the fields of a message structure. Usually you want these autogenerated from a *.proto* file. The tool string *nanopb_generator.py* accomplishes this.
+
+So a typical project might include these files:
+
+1) Nanopb runtime library:
+    - pb.h
+    - pb_decode.h and pb_decode.c
+    - pb_encode.h and pb_encode.c
+2) Protocol description (you can have many):
+    - person.proto
+    - person.c (autogenerated, contains initializers for const arrays)
+    - person.h (autogenerated, contains type declarations)
+
+Features and limitations
+========================
+
+**Features**
+
+#) Pure C runtime
+#) Small code size (2–10 kB depending on processor)
+#) Small ram usage (typically 200 bytes)
+#) Allows specifying maximum size for strings and arrays, so that they can be allocated statically.
+#) No malloc needed: everything is stored on the stack.
+#) You can use either encoder or decoder alone to cut the code size in half.
+
+**Limitations**
+
+#) User must provide callbacks when decoding arrays or strings without maximum size.
+#) Some speed has been sacrificed for code size. For example varint calculations are always done in 64 bits.
+#) Encoding is focused on writing to streams. For memory buffers only it could be made more efficient.
+#) The deprecated Protocol Buffers feature called "groups" is not supported.
+
+Getting started
+===============
+
+For starters, consider this simple message::
+
+ message Example {
+    required int32 value = 1;
+ }
+
+Save this in *example.proto* and run it through *nanopb_generate.py*. You
+should now have in *example.h*::
+
+ typedef struct {
+    int32_t value;
+ } Example;
+ 
+ extern const pb_field_t Example_fields[2];
+
+Now in your main program do this to encode a message::
+
+ Example mymessage = {42};
+ uint8_t buffer[10];
+ pb_ostream_t stream = pb_ostream_from_buffer(buffer, sizeof(buffer));
+ pb_encode(&stream, Example_fields, &mymessage);
+
+After that, buffer will contain the encoded message.
+The number of bytes in the message is stored in *stream.bytes_written*.
+You can feed the message to *protoc --decode=Example example.proto* to verify its validity.
+
+Library reference
+=================
+
+**Encoding**
+
+**Decoding**
+
+**Specifying field options**
+
+**Generated code**
+
+Wishlist
+========
+#) A specialized encoder for encoding to a memory buffer. Should serialize in reverse order to avoid having to determine submessage size beforehand.
+#) A cleaner rewrite of the source generator.
+#) Better performance for 16- and 8-bit platforms.

docs/lsr.css

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+/*
+Author: Peter Parente
+Date: 2008/01/22
+Version: 1.0 (modified)
+Copyright: This stylesheet has been placed in the public domain - free to edit and use for all uses.
+*/
+
+body {
+  font: 100% sans-serif;
+  background: #ffffff;
+  color: black;
+  margin: 2em;
+  padding: 0em 2em;
+}
+
+p.topic-title {
+  font-weight: bold;
+}
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+table.docinfo {
+  text-align: left;
+  margin: 2em 0em;
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+  font-weight: bold;
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+  margin-bottom: 0em;
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pb.h

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 #define pb_packed
 #endif
 
-/* Wire types */
+/* Wire types. Library user needs these only in encoder callbacks. */
 typedef enum {
     PB_WT_VARINT = 0,
     PB_WT_64BIT  = 1,