update problem matchers doc for fromPath and default severity (#256)

Author: ericsciple

docs/problem-matchers.md

@@ -1,13 +1,17 @@
 # Problem Matchers
+
 Problem Matchers are a way to scan the output of actions for a specified regex pattern and surface that information prominently in the UI. Both [GitHub Annotations](https://developer.github.com/v3/checks/runs/#annotations-object-1) and log file decorations are created when a match is detected.
 
 ## Single Line Matchers
 
 Let's consider the ESLint compact output:
+
 ```
 badFile.js: line 50, col 11, Error - 'myVar' is defined but never used. (no-unused-vars)
 ```
+
 We can define a problem matcher in json that detects input in that format:
+
 ```json
 {
     "problemMatcher": [
@@ -33,31 +37,34 @@ The following fields are available for problem matchers:
 
 ```
 {
-    owner: An ID field that can be used to remove or replace the problem matcher. **required**
+    owner: an ID field that can be used to remove or replace the problem matcher. **required**
+    severity: indicates the default severity, either 'warning' or 'error' case-insensitive. Defaults to 'error'
     pattern: [
         {
-            regexp: The regex pattern that provides the groups to match against **required**
+            regexp: the regex pattern that provides the groups to match against **required**
             file: a group number containing the file name
+            fromPath: a group number containing a filepath used to root the file (e.g. a project file)
             line: a group number containing the line number
             column: a group number containing the column information
             severity: a group number containing either 'warning' or 'error' case-insensitive. Defaults to `error`
             code: a group number containing the error code
             message: a group number containing the error message. **required** at least one pattern must set the message
-            loop: loops until a match is not found, only valid on the last pattern of a multipattern matcher
+            loop: whether to loop until a match is not found, only valid on the last pattern of a multipattern matcher
         }
     ]
 }
 ```
 
-
 ## Multiline Matching
 Consider the following output:
+
 ```
 test.js
   1:0   error  Missing "use strict" statement                 strict
   5:10  error  'addOne' is defined but never used             no-unused-vars
 ✖ 2 problems (2 errors, 0 warnings)
 ```
+
 The file name is printed once, yet multiple error lines are printed. The `loop` keyword provides a way to discover multiple errors in outputs. 
 
 The eslint-stylish problem matcher defined below catches that output, and creates two annotations from it.
@@ -94,12 +101,15 @@ The first pattern matches the `test.js` line and records the file information. T
 The second pattern loops through the remaining lines with `loop: true` until it fails to find a match, and surfaces these lines prominently in the UI.
 
 ## Adding and Removing Problem Matchers
+
 Problem Matchers are enabled and removed via the toolkit [commands](commands.md#problem-matchers).
 
 ## Duplicate Problem Matchers
+
 Registering two problem-matchers with the same owner will result in only the problem matcher registered last running.
 
 ## Examples
+
 Some of the starter actions are already using problem matchers, for example:
 - [setup-node](https://github.com/actions/setup-node/tree/master/.github)
 - [setup-python](https://github.com/actions/setup-python/tree/master/.github)