Add "How to run Infection only for changed files" article

src/guide/how-to.md

@@ -0,0 +1,56 @@
+---
+title: How-to Guides
+type: guide
+order: 9
+---
+
+## How to run Infection only for changed files
+
+If you have thousands of files and too many tests, running Mutation Testing can takes hours for your project. In this case, it's very convenient to run it only for the modified files.
+
+Assuming you are on a feature branch, and the main branch is `master`, we can do it as the following:
+
+```bash
+CHANGED_FILES=$(git diff origin/master --diff-filter=AM --name-only | grep src/ | paste -sd "," -);
+INFECTION_FILTER="--filter=${CHANGED_FILES} --ignore-msi-with-no-mutations";
+
+infection --threads=4 $INFECTION_FILTER
+```
+
+The `--diff-filter=AM` returns only added and modified files, because we are not going to use removed ones.
+
+The [`--ignore-msi-with-no-mutations` option](/guide/command-line-options.html#ignore-msi-with-no-mutations) tells Infection to not error on min MSI when we have `0` mutations.
+
+#### Example for Travis CI:
+
+```bash
+jobs:
+  include:
+    - stage: Mutation Testing
+    script:
+      - |
+        if [[ "${TRAVIS_PULL_REQUEST}" == "false" ]]; then
+            INFECTION_FILTER="";
+        else
+            git remote set-branches --add origin $TRAVIS_BRANCH;
+            git fetch;
+            CHANGED_FILES=$(git diff origin/$TRAVIS_BRANCH --diff-filter=AM --name-only | grep src/ | paste -sd "," -);
+            INFECTION_FILTER="--filter=${CHANGED_FILES} --ignore-msi-with-no-mutations";
+
+            echo "CHANGED_FILES=$CHANGED_FILES";
+        fi
+
+        infection --threads=4 --log-verbosity=none $INFECTION_FILTER
+```
+
+For each job, Travis CI fetches only tested branch: 
+
+```bash
+git clone --depth=50 --branch=feature/branch
+```
+
+That's why we need to fetch `$TRAVIS_BRANCH` as well to make a `git diff` possible. Otherwise, you will get an error:
+
+```bash
+fatal: ambiguous argument 'origin/master': unknown revision or path not in the working tree.
+```