add: cheatsheets for the first two sections

02-write-your-first-program/README.md

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+# Cheatsheet: Write your First Go Program
+
+Hi! For reference, you can store this cheatsheet after you take the lectures in this section.
+
+You can also print this cheatsheet and then follow the video lectures in this section along with it.
+
+Enjoy!
+
+---
+
+## COMMAND LINE COMMANDS:
+
+* Enter into a directory: `cd directoryPath`
+
+* **WINDOWS:**
+
+    * List files in a directory: `dir`
+
+* **OS X & LINUXes:**
+
+    * List files in a directory: `ls`
+
+---
+
+## BUILDING & RUNNING PROGRAMS:
+
+* **Build a Go program:**
+
+    * While inside a program directory, type:
+        * `go build`
+
+* **Run a Go program:**
+
+    * While inside a program directory, type:
+        * `go run main.go`
+
+<br /><br /><br /><br />
+_See the next page..._
+
+<div style="page-break-after: always;"></div>
+
+## WHAT IS $GOPATH?
+
+* _$GOPATH_ is an environment variable which points to a directory where the downloaded and your own Go files are stored.
+
+    * **On Windows**, it's in: `%USERPROFILE%\go`
+
+    * **On OS X & Linux**, it's in: `~/go`
+
+    * **NOTE:** Never set your GOPATH manually. It's by default under your users directory.
+
+* **GOPATH has 3 directories:**
+
+    * **src:** Contains the source files for your own or other downloaded packages. You can build and run programs while in a program directory under this directory.
+
+    * **pkg:** Contains compiled package files. Go uses this to make the builds (compilation & linking) faster.
+
+    * **bin:** Contains compiled and executable Go programs. When you call go install on a program directory, Go will put the executable under this folder.
+
+        * _You might want to add this to your `PATH` environment variable if it's not there already._
+
+---
+
+## WHERE YOU SHOULD PUT YOUR SOURCE FILES?
+
+* `$GOPATH/src/github.com/yourUsername/programDirectory`
+
+* **Example:**
+
+    * My github username is: inancgumus
+
+    * So, I put all my programs under: `~/go/src/github.com/inancgumus/`
+
+    * And, let's say that I've a program named hello, then I put it under this directory: `~/go/src/github.com/inancgumus/hello`
+
+<br /><br /><br /><br />
+_See the next page..._
+
+<div style="page-break-after: always;"></div>
+
+## FIRST PROGRAM
+
+* Under, `~/go/src/github.com/inancgumus/hello`, create a `main.go` file.
+* And add the following code to it.
+* Then, run it like this: `go run main.go`
+
+```go
+package main
+
+import "fmt"
+
+func main() {
+    fmt.Println("Hi! I want to be a Gopher!")
+}
+```
+
+That's all! Enjoy!
+
+---
+
+## NOTE:
+
+* BTW, There's a new *Go Modules* support which allows you to run your programs in any directory that you want. After the section ends, you'll also find an information about it.
+
+> For more tutorials: [https://blog.learngoprogramming.com](https://blog.learngoprogramming.com)
+> 
+> Copyright © 2018 Inanc Gumus
+> 
+> Learn Go Programming Course
+> 
+> [Click here to read the license.](https://creativecommons.org/licenses/by-nc-sa/4.0/)