Add files for contributing to the project

Signed-off-by: Miquel Sabaté Solà <[email protected]>

.github/ISSUE_TEMPLATE/custom.md

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+---
+name: Custom issue template
+about: General issues, both bugs and features.
+title: ''
+labels: ''
+assignees: mssola
+---
+
+### Description
+
+- **Expected behavior**: I expected this to happen!
+- **Actual behavior**: But this happened...
+- **Provide the versions** of:
+  - This kernel (git commit sha).
+  - Toolchain version: gcc, cross compiler et. al.
+  - Version of OpenSBI.
+  - Version of QEMU or identifier of the board being used. 

.github/PULL_REQUEST_TEMPLATE.md

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+Provide a general description of the changes in your pull request. If this pull
+request fixes a known issue, please tag it as well (e.g.: `Fixes #1`). In any
+case:
+
+- [ ] Write a [good commit message](https://chris.beams.io/posts/git-commit/).
+      Extra points if it's in the style of the [Linux
+      kernel](https://docs.kernel.org/process/submitting-patches.html).
+- [ ] Make sure that `make all` continues to work.
+- [ ] Make sure that `make lint` does not report any issues.
+- [ ] The pull request has *only* one subject and a clear title. You are not
+      submitting a pull request with tons of different unrelated commits.
+
+Thanks!

CONTRIBUTING.md

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+## Why?
+
+Do you want to fix an error you have found? Do you know a way to improve my
+6502-fu or do you know a technique on NES development that might help here? I am
+open for discussion and welcome any help!
+
+## How?
+
+There are many ways to help me out. One way might be to open an issue on
+[Github's tracker](https://github.com/mssola/list.nes/issues) and start a
+discussion. For this, mind the following:
+
+- Check that the issue has not already been reported or fixed in `main`.
+- Try to be concise and precise in your description.
+- If you have found a problem, provide a step by step guide on how to reproduce it.
+- Provide the version you are using (git commit SHA), as well as the version of
+  the toolchain and the emulator/system being used.
+
+Another way is to simply submit a pull request. For this, also mind these:
+
+- Write a [good commit message](https://chris.beams.io/posts/git-commit/).
+- You are sure that `make` continues to work.
+- The pull request has *only* one subject and a clear title. You are not
+  submitting a pull request with tons of different unrelated commits.

SECURITY.md

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+# Security Policy
+
+This is not a serious kernel, and hence we do not have any serious security
+policy. There are of course many things that could compromise a toy kernel such
+as this. For example:
+
+- Bad bound checks that could turn into a vector of attacks.
+- Bad enforcements of memory protection.
+  - Remember that this kernel does not implement any memory protection on the
+    purpose of being simple, but we could always enable other mechanisms to keep
+    things "secure".
+- Other kind of bugs that could compromise such as kernel.
+
+For any of these situations, since this is a toy project, simply submit an
+[issue to the Github repository](https://github.com/mssola/fbos/issues).