add issue templates to promote decent format of bug reports

.github/ISSUE_TEMPLATE/bug.md

+---
+name: Error/crash/problem report
+about: Something that should work but does not, or produces wrong results.
+---
+
+## Minimal code example to reproduce the problem
+
+> Include a reproducible sample of code that causes the problem. Others should
+> be able to reproduce your problem by just copypasting your code into Julia
+> shell. Use `download` to get any necessary datasets from the internet.
+
+```
+using SBML
+download("https://badmodels.org/model1.xml", "model.xml")
+readSBML("model.xml")
+```
+
+## Expected behavior
+
+> Describe what you expected to happen if the above code worked correctly, and
+> why you think that behavior is correct.
+
+## Actual behavior
+
+> What was the unexpected thing that happened instead?
+
+## Optional: Suggestions for fixing
+
+> Feel free to suggest whatever change in the package that might remove the
+> described problematic behavior. You may want to open a pull request if you
+> already have a fix.

.github/ISSUE_TEMPLATE/enhancement.md

+---
+name: Feature request
+about: Suggest new things to be implemented in the package
+---
+
+## Target functionality
+
+> Write a (pseudo)code example that you would like to work. If there is a
+> dataset that you want to load in a new way (e.g., parse out new data from
+> `libsbml`), include a link to the dataset.
+
+## Desired output
+
+> Suggest what would be the optimal output of the above "target" command for
+> you, or how the newly loaded data should be represented by SBML.jl.
+
+## Optional: Suggestions for implementation
+
+> Feel free to point out a viable way to add the functionality, including
+> pointers to required API functions and ideas about new data structures.
+> If you have a code that implements the feature (at least partially), you may
+> want to open a pull request instead.