feature/pr-template (#37) (#38)

### Description
Recently the project received a PR which was made to master, but it should be made to dev first. This PR add a common explanation of how to create a PR.

### Checklist
- [x] All methods associated with structs has ```func (self *struct) method() {}``` name style.
- [x] Executed ```test.sh``` and updated the [README.md](README.md) with new code coverage data.
- [x] Wrote unit tests for new/changed features.

README.md

@@ -1,4 +1,4 @@
-# Maroto [![GoDoc](https://godoc.org/github.com/johnfercher/maroto?status.svg)](https://godoc.org/github.com/johnfercher/maroto) [![Travis](https://travis-ci.com/johnfercher/maroto.svg?branch=master)][travis] [![Code Coverage](https://img.shields.io/badge/coverage-95.5%25-brightgreen.svg)][test] [![Go Report Card](https://goreportcard.com/badge/github.com/johnfercher/maroto)](https://goreportcard.com/report/github.com/johnfercher/maroto)
+# Maroto [![GoDoc](https://godoc.org/github.com/johnfercher/maroto?status.svg)](https://godoc.org/github.com/johnfercher/maroto) [![Travis](https://travis-ci.com/johnfercher/maroto.svg?branch=master)][travis] [![Code Coverage](https://img.shields.io/badge/coverage-95.1%25-brightgreen.svg)][test] [![Go Report Card](https://goreportcard.com/badge/github.com/johnfercher/maroto)](https://goreportcard.com/report/github.com/johnfercher/maroto)
 A Maroto way to create PDFs. Maroto is inspired in Bootstrap and uses [Gofpdf](https://github.com/jung-kurt/gofpdf). Fast and simple.
 
 > Maroto definition: Brazilian expression, means an astute/clever/intelligent person.

pull_request_template.md

@@ -0,0 +1,13 @@
+### Disclaimer
+Please, do not create a Pull Request directly to master branch, create to dev always.
+
+### Description
+Please, describe how this PR will be useful. If it has any tricky technical detail, please explain too.
+
+### Related Issue
+If it has any issue related to this PR, please add a reference here.
+
+### Checklist
+- [ ] All methods associated with structs has ```func (self *struct) method() {}``` name style.
+- [ ] Executed ```test.sh``` and updated the [README.md](README.md) with new code coverage data.
+- [ ] Wrote unit tests for new/changed features.