docs: add recipe for linting of all commits in a PR

* be more explicit / guide more than #24
* closes #35
* connected to #12

Author: marionebl

.conventional-changelog-lintrc

@@ -67,6 +67,16 @@ As part of `npm test`
   }
 ```
 
+#### Lint all commits in PR
+
+```sh
+# On Travis CI
+conventional-changelog-lint --from=$TRAVIS_BRANCH to=$TRAVIS_PULL_REQUEST_BRANCH
+
+# E.g.:
+conventional-changelog-lint --from=master to=docs/add-pr-recipe
+```
+
 #### Travis
 
 ```yml
@@ -208,7 +218,7 @@ Perform `git fetch --shallow` before linting.
 Most likely you are reading this because you where presented with an error message:
 
 ```
-  'Could not get git history from shallow clone. 
+  'Could not get git history from shallow clone.
   Use git fetch --shallow before linting.
   Original issue: https://git.io/vyKMq\n Refer to https://git.io/vyKMv for details.'
 ```
@@ -217,7 +227,7 @@ Most likely you are reading this because you where presented with an error messa
 
 git supports checking out `shallow` clones of a repository to save bandwith in times.
 These limited copies do not contain a full git history. This makes `conventional-changelog-lint`
-fail, especially when running on large commit ranges. 
+fail, especially when running on large commit ranges.
 To ensure linting works every time you should convert a shallow git repo to a complete one.
 Use `git fetch --shallow` to do so.
 
@@ -226,7 +236,7 @@ Use `git fetch --shallow` to do so.
 Ensure full git checkouts on TravisCI, add to `.travis.yml`:
 
 ```yml
-before_install: 
+before_install:
   - git fetch --unshallow
 ```