Add vision and design principles doc

VISION.md

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+# Vision and Design Principles
+
+## Vision
+
+Lazygit's vision is to be the most enjoyable UI for git.
+
+## Design Principles
+
+There are seven (sometimes contradictory) design principles we follow:
+
+- Dicoverability
+- Simplicity
+- Safety
+- Power
+- Speed
+- Comformity with git
+- Think of the codebase
+
+### Discoverability
+
+TUI's are notoriously hard to learn, thanks to limited screen real-estate to provide contextual help and a general lack of effort on the part of developers to make things obvious. We want Lazygit to buck the trend and be easy for a new user to grok.
+
+Examples:
+
+- Clearly document all the features/configuration options
+  - e.g. gifs in the README
+- Document how to solve various git problems with Lazygit
+  - This is something we don't have yet but should: a section in the docs explaining how Lazygit can help you in various scenarios
+- Use tooltips to explain what actions will do
+- Make it easy for users to ask questions and get answers from the community
+- Make it easy to find entities and actions from within Lazygit
+- Use visual elements to make things obvious
+  - e.g. '<-- YOU ARE HERE' label when rebasing
+- Don't require the user to memorise keybindings
+  - e.g. when the user is mid-rebase, we prominently show that the keybinding for viewing rebase options is 'm'
+- When the user performs an action in Lazygit, make the impact obvious
+  - If the affected entity isn't visible, show a toast notification
+- If a keybinding is disabled, give a reason why
+
+### Simplicity
+
+The git CLI is very complex but most git use cases are simple. Lazygit needs to ensure that simple use cases are easy to satisfy.
+
+- Make the most common use cases dead-simple (staging files, committing, pulling/pushing)
+- Don't overwhelm the user with options
+- Use sensible defaults
+- We already have too many configuration options: think hard before adding any new ones
+
+### Safety
+
+It's easy to screw things up in git so Lazygit should try to protect the user from screwing things up.
+
+- Prompt for a confirmation before doing anything that's hard to reverse
+- Make it easy to correct mistakes
+  - e.g. undo action
+  - the escape key should get you out of most transient situations (rebasing, diffing, etc)
+
+## Power
+
+Users shouldn't have to drop down the CLI _too_ often. Lazygit should be able to handle some complex use cases.
+
+- Make complex (but common) CLI flows simple
+  - e.g. interactive rebasing
+- Use the custom commands system to handle the really rare complex edge-cases
+
+### Speed
+
+Pro users should be able to move at lightning speed with Lazygit.
+
+- Always think about the number of keypresses involved in a given UX flow
+- Make lazygit performant and responsive
+- Think about the individual commands being run and how fast they are
+- Startup should be FAST. If you want to run something at startup that is slow, make it non-blocking.
+- Support muscle-memory
+  - Prefer disabling menu items instead of hiding them so that muscle memory can be used to select the desired menu item
+  - Try to make keybinding intuitions to transfer across contexts (e.g. 'd' for destroy)
+  - When changing keybindings in a new release, always consider what will happen if a user does not read the release notes and relies on muscle memory.
+
+### Conformity with git
+
+Satisfying the use-cases of git users is more important than perfectly conforming to git's API, but even obscure parts of git's API were motivated by real use-cases.
+
+- Users should only have to drop down to the git CLI in rare circumstances
+- Honour the git config
+  - Don't override anything set in the git config without the user's permission
+- Work with git, not against it.
+  - Too much magic will get us into trouble
+- Avoid storing Lazygit-specific session state that could instead be stored in git
+- Ensure that Lazygit can represent the state of any repo
+- Sometimes git's default behaviour is just silly and we'll make the call to override but it should be a well-considered decision.
+
+### Think of the codebase
+
+Will somebody PLEASE think of the codebase!
+
+Some features are not worth the added complexity in the codebase. The more this codebase grows, the harder it will be to make the changes that everybody wants.
+
+## Resolving conflicts
+
+Many of the above objectives are directly antithetical to one another. If you add an extra confirmation prompt for the sake of _safety_, you're sacrificing _speed_. If you support toggling various git flags in the name of _power_, you're sacrificing _simplicity_. There are a few things to say here.
+
+When there are conflicts, we need to make a judgement call. In general we should err on the side of safety and simplicity as the default, with the ability for users to make things faster / more powerful either through configuration or separate keybindings.
+
+This does not mean for example that force pushes should be impossible without being manually enabled: force pushes are table stakes for anybody who rebases. But it does mean that a confirmation popup should appear when force pushing.