Update issue templates

Author: dpvc

.github/ISSUE_TEMPLATE/bug_report.md

@@ -1,6 +1,6 @@
 ---
 name: Bug report
-about: Create a report to help us improve
+about: Create a report to help us improve MathJax
 title: ''
 labels: ''
 assignees: ''
@@ -13,4 +13,4 @@ https://github.com/mathjax/MathJax/issues
 
 unless they are about structural issues in this repository (e.g., the package.json file, the compilation process, the repository layout, etc).  Bugs with how MathJax works should be reported at the link above.
 
-If you are not part of the MathJax team, it is unlikely that you should be posting an issue here.  Use the tracker linked above.
+If you are not part of the MathJax team, it is unlikely that you should be posting an issue here.  Use the tracker linked above instead.

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,16 @@
+---
+name: Feature request
+about: Suggest an idea for MathJax
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+Please submit MathJax feature requests at
+
+https://github.com/mathjax/MathJax/issues
+
+unless they are about structural issues in this repository (e.g., the package.json file, the compilation process, the repository layout, etc).  New features for MathJax should be requested at the link above.
+
+If you are not part of the MathJax team, it is unlikely that you should be posting an issue here.  Use the tracker linked above instead.

.github/ISSUE_TEMPLATE/mathjax-source-problem.md

@@ -1,6 +1,6 @@
 ---
 name: MathJax Source Problem
-about: Instructions for Reporting Issues
+about: Suggest improvements to the source respository
 title: ''
 labels: ''
 assignees: ''
@@ -9,4 +9,4 @@ assignees: ''
 
 This issue tracker is for problems with the source code repository (e.g., its structure, package file, compilation process, organization, etc.), not for problems with the MathJax code itself.  If you are reporting a bug in how MathJax works, please use the [MathJax issue tracker](https://github.com/mathjax/MathJax/issues), rather than this one, so that all the issues are in one place for easier searching and sorting.
 
-If you are not part of the MathJax team, it is unlikely that you should be posting an issue here.  Use the tracker linked above.
+If you are not part of the MathJax team, it is unlikely that you should be posting an issue here.  Use the tracker linked above instead.