doc: improve documentation on how to use, command description and syntax

Author: dygufa

README.md

@@ -8,12 +8,51 @@ The sdkgen is a tool that aims on solving client-server communication and data l
 
 ## How to use it
 
+1) Install Crystal following instructions from: https://crystal-lang.org/docs/installation/;
+2) Download this repository;
+3) Run sdkgen in order to get the desired `sdk`. For more details about the command, check the next section:
 `crystal run main.cr -- api.sdkgen -o api.ts -t typescript_nodeserver`
 
-
-## TODO
-
-- [ ] Improve README
-- [ ] Create and host documentation page
-- [ ] Make it database neutral
-- [x] Move associated tooling related to package publishing to travis
+## Options
+
+`crystal run main.cr -- <SDKGEN FILE> -o <OUTPUT FILE> -t <TARGET>`
+
+- `<SDKGEN FILE>`: .sdkgen file containing the API contract, check the next section to see more details about the syntax;
+- `<OUTPUT FILE>`: filename for the generated file;
+- `<TARGET>`: the target platform for where you are generating the sdk. Options avaiable are:
+    - `java_android`
+    - `kt_android`
+    - `swift_ios`
+    - `typescript_nodeclient`
+    - `typescript_nodeserver`
+    - `typescript_servertest`
+    - `typescript_web`
+
+## Syntax
+
+The .sdkgen file is responsible for declaring the API contract and for that purpose it implements the following syntax:
+
+```
+type Message {
+    date: date
+    author: User
+    mentions: User[]
+    text: string?
+}
+
+// Sends a message, returns nothing
+function sendMessage(target: string, message: Message): void
+```
+
+The primitive types available are:
+
+- `bool`: Either `true` or `false`
+- `int`: an integer from -2147483648 to 2147483647
+- `uint`: a positive integer, from 0 to 4294967295
+- `float`: a floating-point number, equivalent to a `double` or `float64` from other languages
+- `string`: an UTF-8 encoded sequence of characters. This is not meant to store binary data, only printable/readable characters. There is a soft limit of 65535 chars in a string.
+- `date`: A point in time with millisecond precision. Timezone information is not preserved and will be converted to the local timezone of the receiver always.
+- `bytes`: Arbitrary binary data.
+- `void`: Special type that means the lack of value. Can only be used as the return type of a `function` operation (more on that later)
+
+You can check the complete description on [language.md](../blob/master/language.md).