Explain prepare as part of load and run, and update gitignore (#115)

* Explain prepare as part of load and run and update gitignore

Explains that the prepare function is called during the load and run phase.
Adds generated logs and result json files to gitignore.

.gitignore

@@ -1,4 +1,5 @@
 /target
 *.iml
+latte-*
 .idea
 .vscode

README.md

@@ -159,8 +159,10 @@ pub async fn schema(ctx) {
 ### Prepared statements
 
 Calling `ctx.execute` is not optimal, because it doesn't use prepared statements. You can prepare statements and
-register them on the context object in the `prepare`
-function:
+register them on the context object in the `prepare` function. 
+They will be executed during [the load step](#populating-the-database) before the actual database population 
+and the run step before executing the run function.
+An example of implementing and using prepare:
 
 ```rust
 const INSERT = "my_insert";